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Whining noise from front, engine speed-related

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Can anyone offer advice on this please. The car is a recent purchase, 2002 4S Coupe with 92k miles. Drove fine for the first month, but now a whining noise sounds as if it’s is coming from somewhere behind the dash/firewall/front axle area. The whine varies with engine speed/gear selection, so it presumably can’t be from the tiptronic transmission or front axle otherwise it would vary with road speed. However, the noise completely disappears when the a/c blower is turned off. I’m not great on electrics but was wondering if the blower fan might be failing and needing more alternator output at higher revs? I haven’t noticed anything untoward on the voltmeter. The a/c needs a re-gas, but I can’t think that can have any relevance? Any thoughts much appreciated.

Edited by SteveTTT
Typo correction
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I think I’ve now ruled out the blower fan. With the engine off the blower is as smooth and quiet as you like, at all fan speeds. With engine running and a/c off it’s fine too, but switch on the a/c and after a few seconds a new slightly harsher sound becomes apparent. (All this is with the car stationary). I’ve checked out the cooling fans and they seem fine, so I’m now thinking it could be noise transmitted from elsewhere, maybe the a/c compressor - that would account for the varying pitch of the whine when I’m on the move. Have run out of time today, but I think the next step is to look in the engine bay before I have the a/c re-charge is done. 

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Spot on creekman, that’s just the kind of stethoscope I was thinking of. Simply listening to the various engine noises doesn’t raise any concerns, but it’s all a bit loud back there! However, I just spoke to a friend at my local country pub, he’s a Mercedes specialist for his sins, who says the “evaporator” can make weird noises when the a/c system pressure is too low. I think his evaporator might equate to dessicator/receiver-dryer in Porsche-speak and my original view that the noise came from the firewall area might tie in with the location of that bit of kit. So your original suggestion of doing the re-gas first is what I think I’ll now do. I’ll feedback when I’ve done that. Cheers, Steve

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  • 2 months later...

It’s sorted! One condenser was replaced and the system re-gassed; no more problems. And well done to Porsche independent Brookspeed in Hampshire UK for sorting this out without quibble on a customer’s car sold on brokerage. They also sourced a new remote key f.o.c. Great customer service.

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That’s correct JTT. I haven’t identified exactly how the noise was being created, but low (or no) pressure in the system was the cause. I mentioned earlier in the thread that a Mercedes specialist friend of mine had experienced something similar on Mercs.

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Mine had no cooling whatsoever JJT. I’m no a/c expert but I think if there was a leak the system would get depleted very quickly. Is there some kind of pressure relief valve in the system that might be malfunctioning? (Just a stab in the dark!). As creekman said earlier on, maybe a pressure test and recharge is a relatively low cost way of eliminating potential issues? Hope you get it sorted.

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I guess you really don't need A/C, but if you lived in Texas you do or melt...  I've had the insulation problem for some time and now I guess there's no more left.  My A/C is slow to get going, once it does it does a good job.  If I put it on the recirculating mode the A/C cools down quickly.  I'm not sure if this is part of the vent door problem or another issue?

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