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toddgibbs123
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Joined: 23 May 2015
Posts: 25
Location: Oxford, UK


PostPosted: Wed Mar 15, 2017 11:09 am    Post subject: Strange resonant hum - HELP! Reply with quote

hi Guys,

Posted this on the PorscheGB forum so sorry if you have already seen this.

I have a strange resonant hum in my car that I cannot track down.

Here is a video of it https://www.youtube.com/watch?v=aJF5deGvKQA

It is not engine rpm dependent, vehicle static or move, not the A/C, not the fans, not the fuel tank resonating. I cannot hear it outside the vehicle, and cannot tell if its front or back.

What could it be and how do I track it down? It actually makes me feel a little sick after a while - and annoyingly its loudest right where I put my head (notice is faces as I move the camera around).

Thanks for any help!

Todd
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Robertb
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Joined: 01 Sep 2003
Posts: 5959
Location: South Oxfordshire

2002 Porsche 996 Carrera 4S

PostPosted: Wed Mar 15, 2017 11:36 am    Post subject: Reply with quote

I have a noise in my car, more of a whine, that is caused by the stereo system. Oddly it is not volume dependent and only goes when the stereo is completely switched off as opposed to standby. Might be worth checking that the stereo is not picking up interference.

In your car, it sounds more like a pump or a bearing. The a/c compressor in the 996s can get a bit grumbly when a bit low on gas, not sure if 993s work in the same way.

You are presumably looking for a pump or motor which runs at a constant speed.
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toddgibbs123
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Joined: 23 May 2015
Posts: 25
Location: Oxford, UK


PostPosted: Wed Mar 15, 2017 11:55 am    Post subject: Reply with quote

Thanks Robert,

Its definitely not the stereo.

Yes, could be a bearing/pump (not sure how to identify which/where?) - I semi-suspect a vacuum hose...

Todd.
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Robertb
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Location: South Oxfordshire

2002 Porsche 996 Carrera 4S

PostPosted: Wed Mar 15, 2017 12:10 pm    Post subject: Reply with quote

I would take it to a specialist...they will know which pumps and bearings are not engine RPM dependent, or hoses as you suggest. At one point in your video you can hear it slightly change in pitch even though the engine is at idle, which leads me to think the noise source it has its own power supply.

Who do you use for service... Wrightune?

These things are a nightmare once they get in your head! Sounds lovely apart from the hum...
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Endoman
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Joined: 11 Apr 2011
Posts: 755
Location: Bolton U.K.


PostPosted: Wed Mar 15, 2017 3:02 pm    Post subject: Reply with quote

Is it there with ignition on, not running? Then it's electrical. The little fan in the hvac can make a liitle noise but not like that. Check the DME relay it may be vibrating and the fuse box acting as a resonator. Sure it's not the fuel pump or a fuel line come adrift? Or even, if running, one of the sill pipes, oil or a/c clamp broken and vibrating against the floor pan? Heater fan bearings or a servo in one of the flap controllers?
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toddgibbs123
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Joined: 23 May 2015
Posts: 25
Location: Oxford, UK


PostPosted: Wed Mar 15, 2017 3:12 pm    Post subject: Reply with quote

Endoman wrote:
Is it there with ignition on, not running? Then it's electrical. The little fan in the hvac can make a liitle noise but not like that. Check the DME relay it may be vibrating and the fuse box acting as a resonator. Sure it's not the fuel pump or a fuel line come adrift? Or even, if running, one of the sill pipes, oil or a/c clamp broken and vibrating against the floor pan? Heater fan bearings or a servo in one of the flap controllers?


Thanks Endoman

Its only there with the motor actually running. Its not the Hvac (if thats the little fan on the back of the CCU).

Will check the DME/fusebox area.

I suspected the fuel pump as a random guess as its quite central to the car (I believe the fuel pump is roughly beneath where me). On my 930 I could trick the system to run the fuel pumps without the motor started... is that possible/allowed/crazy on a 993 ?

It doesn't sound like something physically banging against something else. Its too high-pitched.

It isn't the heater fan (its still there with heater fan off, and I've pulled the rear blower fuse and its still there). My LHS front footwell servo has failed - maybe its that?

Thanks for the help Smile

Todd.
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Demort
Magny-Cours


Joined: 21 Mar 2015
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Location: Sussex


PostPosted: Wed Mar 15, 2017 6:55 pm    Post subject: Reply with quote

It does sound like a bearing / whine and if not rpm related then all i can think of is the fuel pump .. im wondering if the pump or a pipe is touching the body work .

Nice few picture here on the pump and the support rubbers .

http://rennlist.com/forums/993-forum/812752-993-fuel-pump-diy.html
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Endoman
Imola


Joined: 11 Apr 2011
Posts: 755
Location: Bolton U.K.


PostPosted: Wed Mar 15, 2017 9:41 pm    Post subject: Reply with quote

re fuel pump yes you can run it by pulling DME relay and jumping terminals 30 and 87b ign off. It's the same relay on my 944 and I found a bad check valve that way which was causing poor hot starts.
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toddgibbs123
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Joined: 23 May 2015
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PostPosted: Thu Mar 16, 2017 12:37 pm    Post subject: Reply with quote

Thanks Gents, will post back once I have tried all suggestions.

Todd.
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toddgibbs123
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Joined: 23 May 2015
Posts: 25
Location: Oxford, UK


PostPosted: Mon Mar 20, 2017 1:07 pm    Post subject: update. Reply with quote

Ok, I had the cover off of the fuel pump over the weekend. Everything seems secure and I don't think the noise is coming from there.

It does seem to be coming from the rear of the car (but its very difficult to tell has its one of those low resonant tones).

What is back there that doesn't depend on engine rpm? I already tried pulling the rear blower fuse...

Thanks again for help

Todd.
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9xxnick
Silverstone


Joined: 29 Jul 2014
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PostPosted: Wed Mar 22, 2017 7:27 pm    Post subject: Reply with quote

Has the car always done this in your ownership? If not, was anything at all changed or worked on before it started? If not, have you considered contacting the previous owner and seeing if he/she knows anything about it? (Oh yes! That started after I re-routed the oil pipes...")

One other possibility is going through the fuse box and removing each fuse one at a time to see if that shows the component/circuit that's the culprit. Of course it may not be electrical, and the fact that it isn't present when the ignition's switched on but the engine's not running probably supports this, but it's possible there's a circuit that only switches in when a demand is placed on it.

One other thought - does the car have the amplifier from the hifi pack installed (assuming that was fitted as an option)? Could that still be being powered up even if the stereo's disconnected?

Good luck with finding the cause.
 
  
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Demort
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Joined: 21 Mar 2015
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PostPosted: Wed Mar 22, 2017 8:01 pm    Post subject: Reply with quote

Get the power steering system checked .. im thinking its a pipe that is touching the bodywork and transmiting the noise .. pas is not rpm dependant its a constant pressure so could very well cause a noise like this .
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toddgibbs123
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Joined: 23 May 2015
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Location: Oxford, UK


PostPosted: Thu Mar 23, 2017 12:48 pm    Post subject: Reply with quote

As always, thanks for the suggestions. Will try them out over the weekend and post back.

@9xxnick - I've only had the car since the beginning of the year. I don't think it did it at the very start as I am sure I would have noticed it. I will try systematically pulling fuses (wasn't sure I was ok to do this). The car doesn't have an amp so its not that.

Thanks:)

Todd.
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toddgibbs123
Newbie


Joined: 23 May 2015
Posts: 25
Location: Oxford, UK


PostPosted: Fri Apr 07, 2017 9:51 pm    Post subject: Reply with quote

sorry for slow reply.

Yesterday I systematically removed each fuse in the front fuse box. Made no difference (apart from the obvious fuel pump fuse!). Hum was still there.

I will try the rear fuse box next (tomorrow hopefully) and maybe the relays.

One thing I notice is that its a constant hum when stationary but when driving it does vary, like hhmmmm, hmm, hmmmm, hmmmmm, hmmm. So it seems affected by air flow (?)

Could it be anything like the front oil cooler fan? (although I think that should be off when I first start?) or the carbon canister?

Thanks again

Todd.
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