I've been trying to track down a very strange noise from my car for a few months now and finally think I'm on to something. Tried searching for others who may have had this issue but no luck on here.

Overall, the sound started after I got done doing a decent amt of work on my car including reinstalling the AC. The noise occurs at 2100 RPM and 3000 RPM only and is not gear dependent. Ive been assuming it was mechanical because the noise came from the alternator area so I replaced the alternator, belts and some other bits and pieces. None of that did anything. Tried removing belts to narrow it down and no real luck.

The other day after talking to a mechanic he had an epiphany, maybe the issue was electrical vs mechanical. I then drove the car around and got the car to 2100 RPM. I had the AC on and turned it off by depressing the little AC button on the dash. Low and behold the noise stopped. When I pushed it back on the noise came back immediately. Did this at least a dozen times and same result. I then unhooked the power to the compressor and the noise did not occur. This tells me it is the compressor vs other wiring or at least the compressor load.

Anyone else ever experience this? If so what was the issue and solution. Im thinking I need to replace my compressor but hate to do so if the issue is something else.

Would really appreciate any thoughts or input on the issue

Thanks
Brian

 

Just a guess, but compressor clutch or compressor clutch bearing issue?

 

Brian, I have a very similar issue as you describe. When running the AC I
get a piercing whine at about 3000 rpm. It's hard to tell where it's coming
from but seems to be coming up from the shifter area. Mine is easily reproduced
and is most annoying in 3rd gear which is used a lot. Turning the AC off
will make the sound go away or at least reduce it to a negligible level.

Turning max AC on will give the loudest whine. Regular AC will reduce the
whine and it seems to vary with the fan speed.

I've been living with it since I switched to RS motor mounts. Not sure if
that is a coincidence or is part of the problem.

I also had the alternator replaced back in the winter.

I just finished doing a brake job on my car so now I'm ready to focus on
fixing this problem as it is makes driving very uncomfortable.

I don't have a solution yet so I'm open to ideas to investigate.

Possible components to check: transmission, AC compressor, cabin fan,
motor mounts.

One thing I'm thinking is that harmonics are involved.

Later,

-bruce

 

I have a whine when I use the AC now as well. I replaced the alternator but also replaced engine mounts with RS mounts. I'm putting my money on the RS mounts. I'm guessing the harder rubber in the RS mounts no longer isolate the vibration from the compressor.

 

That's a real bummer if it's the RS mounts. I'm going to do some checking of the AC compressor assembly. Would like to rule it in or out. Seems there can be a few causes of compressor noise. A failing part of the compressor could be masked by the standard motor mounts but revealed by the RS mounts.

 

It could very easily be an issue somewhere in the AC side of the HVAC system, especially if it hadnt been run for a long stint. Anything the refrigerant touches also gets lubricated by the liquid. Thats why its recommended to run the AC in cars that sit for long stretches even when you dont need it on. So I would start with what nine9six said before searching for more exotic causes. Good luck.

 

there is a similar thread on the 964 forum...

here is the link:

https://rennlist.com/forums/964-foru...-with-rev.html

 

I have a new noise that is from the AC when it kicks on (probably compressor, but could be clutch, since its only when it is engaged), so possibly similar to what you are talking about here. Its also sensitive as OP mentions to AC on/off with the button.

I was doing some reading and I've seen other types of cars mention opening up the clutch, cleaning up the plates and possibly removing some shims to decrease the clutch gap as it wears. Anyone try some thing like this on the 993?

If you do have to replace the compressor, don't bother looking at Porsche (you will have a heart attack, end up with a huge hospital bill to match the compressor cost...). Its a Denso unit and I've found it on Amazon, eBay (but more expensive or used), O'Reilly (but random brand) and Pelican (Denso with clutch $313, the Porsche there is only $1800...).

 

This is interesting. I just assumed the whine was related to the RS mounts, so there was nothing I could do about it. After reading the 964 link, I'm wondering if there really is a problem and its related to my new alternator. My whine is present all the time and increases when revved (just like the link in the 964 thread), just more noticeable when the AC compressor kicks in. It's really not loud enough to bother me however.

 

This may sous silly but is your stereo powered on while driving even if you aren't playing any music? A bad ground loop in the stereo wiring can pick up an annoying alternator whine that fluctuates with engine rpm.

I had this issue and fixed it by extending a ground wire from my receiver to the same ground pout under the front passenger seat where the OEM amp is grounded. The noise is gone now.

 

Lot's of good ideas here of possible causes for the whining noise.

I called CAE in Van Nuys who provided the rebuilt alternator to TRE
who installed for me and asked them about the whining noise. The
first thing he said was it means one or more of the diodes is burned
out. Secondarily, the positive cable from the alternator to the battery
could be loose.

The diodes (there are 6) turn AC into DC to charge the battery. If one
is not working, then AC will ripple through the DC. It is best checked
at the B+ terminal on the alternator with a scope using AC coupling.

Here is a video by ScannerDanner showing how it's done:


On the 993 of course the access to the B+ terminal on the alternator
is not that easy. I think the biggest issue I have is doing it safely.

I will need to do this though to rule out the alternator as the cause and to
have it repaired if it is the culprit.

Unless of course someone finds another solution in the meantime...

-bruce

 

Bruce, interesting idea however I can't imagine 3 different alternators would all have the same issue. Im really wondering about the compressor being the issue. As Bruce7 has, mine seems to fluctuate with fan speed when not in max mode. When AC is off and I go through fan speeds there is no noise. Im also wondering if it is a load issue cause by the compressor. If its that I have no idea how to track it down. May have to have my local shop take a look then. I hate to replace the compressor if it isnt that as evacuating and recharging the system is expensive even thought I can swamp the compressor in less than 5 min myself.

 

Took a look at the 964 thread and my issue is different. The noise doesnt fluctuate based on RPM Its always the same pitch and only occurs at certain RPMs. It almost makes me want to remove the AC again to get rid of it but that's a bad idea living in Florida.

 

One test I want to do but I've just not got the time right now is to disconnect
the door speakers and see if the whine is present. If not, then that means
it is RF noise coming in through the speakers. If it is, I would suspect the
air conditioner compressor/clutch.

As soon as I can I also want to check for AC at the battery and if possible at the
alternator, but that may be a challenge. I may be able to see just enough of
a ripple at the battery to diagnose a failed diode.

Right now I don't suspect the air conditioner/clutch. Here is an article I
found listing a number of compressor noises and none of them seem to
match this situation.

http://www.sanden.com/objects/Compressor_Noise.pdf

Here is a article I found about alternator whine with useful information.

http://www.avweb.com/news/maint/182896-1.html

-bruce

 

If you have engine mounts that are stiffer than stock (RS, Wevo, etc), you will get some whine when the AC compressor is on.

I see you've been told several times by experts in previous threads that your RS mounts contribute more NVH to the cabin, but you don't seem to believe it. If you don't like the compressor whine, reinstall the stock engine mounts.