MotoJB

Hey gents,

My 2001 996 TT is making a pronounced whirring/humming noise while under acceleration...a noise that seems to change (increase/decrease in pitch) depending upon RPM. Has a slight "knocking" sound to it as well but I would call it more of a whirring noise.

It doesn't make any such noise at idle, nor does it seem to make any noise when pushing in the clutch (coasting at speed) and/or while revving the engine.

Any idea's what this might be? Concerning.

Thanks.

Atrox

Stereo is still my guess.

MotoJB

Thanks Atrox will certainly look more into that.

Atrox

Mine was doing the same thing, it was driving me insane. Check your amp and ground. RCA lines will pick up feedback depending on ground. Do you have an aftermarket radio?

MotoJB

Even with the stereo off??

Atrox

Yes, my issue turned out to be the radar sensors that were wired to the jammer that was tied into stereo power supply. The kill switch had a bad connection that turned the jammer on/off.

MotoJB

Well, this thing does have an aftermarket kenwood deck, focal speakers...some big alpine amps in the trunk...the sub box was pulled out of the back by the previous owner so the system isn't functioning well at the moment.

Simply have never had a custom after market stereo. Unaware if this could be making such noise only during acceleration, and when the stereo is in "standby" mode or if powered off.


Thanks again, will keep investigating.

MotoJB

Ok, update...pulled the radio/speaker fuses and as expected, the sound is still there.

A bit more clarification - seems the noise is non existent when driving around town. I only seem to hear it when I'm on the freeway (at speeds where I'm using 4th - 6th gears).

Cruising along in 5th or 6th gear on the freeway, it seems to be the loudest between 2200 and 3000 rpm's. Again, every time I step on the gas ever so slightly (even when not in boost - so not a symptom of the Turbos).

It's just a whirring, whining noise that changes in pitch linearly with RPM's.

Macster

The turbo turbines/compressor spin all the time the engine is running even if no boost is being made.

Unless you have done this -- I haven't read all the previous posts -- you have to attempt to determine if the noise is road noise related, tire noise, or wheel bearing noise, drivetrain noise, or engine or engine accessory drive noise.

If you are hearing the noise at speed then step on the clutch and lift off on the gas simultaneously and listen if the noise is still present.

Another test is to at speed and the noise present lift off the accelerator. Often times drivetrain noise especially when related to the diff (or in this case diffs) the noise can be present under acceleration and even steady speed cruising, but go away under closed throttle with the resultant engine braking. Or vice versa.

If it is engine noise you -- or a helper outside of the car -- should be able to hear the noise with the vehicle stationary but of course everything up to temperature.

Or you have to enlist the aid and ear of a trained/experienced tech who can road test the vehicle with you along as a passenger directing him in order to reproduce the sound and let him make the diagnosis.

MotoJB

Thanks Macster...

Sure doesn't seem turbo related nor road noise related.

Stepping on the clutch while at speed on the fwy, the noise goes away. It's only when I'm in gear (4th through 6th cruising on the fwy) and touching the gas to any degree do I hear it.

Don't seem to hear anything strange at all when cruising around town.

911mhawk

I had a whining/whirring noise in 4/5/6 that I hope is not your scenario. Have you pulled the drain plug on trans to find some metal shavings? Refill with Delvac, see if it still makes noise, drive for another couple hundred miles and drain again if you found metal the first time. I found another hairy drain plug and had to rebuild trans. Best case you get fresh lube in trans, worst case you'll need more for your wallet/***, but have a good excuse for a GT2 main/gears and refresh.

MotoJB

Heard it a little bit in 3rd when driving today but not like 4-5-6. Hope it's not what you experienced but will have to check it out. How much did it cost to rebuild your trans btw?

911mhawk

Dump the oil and refill as I described earlier, it takes an hour start to finish if you're going slow and checking out the underside of your ride while at it. if it is grey and watery, it should have been changed sooner. Refill, drive and dump again to check plug and see if looks like mine here. This pic was after my last track day last fall, I checked the date on the pic. Basically a week of commuting and 4 or 5 twenty minute sessions on track generated the shavings. I'm not sure what amount of metal is "normal" but noise that's getting worse under accel/decel, and becoming evident in additional gears is pretty indicative of issues that will only get worse.

I'm pretty certain my desire to drive more at the end of last summer cost me additional rebuild parts, so fix sooner than later, IF YOU CONFIRM IT IS THE TRANSMISSION!
My rebuild cost was more than it needed to be since lots was shot, including the bearing retaining plate from driving it longer than I should have.
I did GT2 main shaft, steel synchros, oiling ports, etc. that drove it up a bit.
What I can think of on the fly:
~$8k in gears/synchros/bearings
Por$che Cup LSD, in addition
+Labor = very reasonable part, thanks Brian Copans
+Cooler/Lines
+UMW/Kevin LFW/clutch kit
+Cup Cables, RS motor mounts, trans mount
The math gets ugly quick if you need lots parts, just look at PET price a few pieces, lots of $300-600 pieces make up these transmissions.
While it's out...water hoses, pin fittings, plugs, coils, reservoir, fuel filter...

911mhawk

Lots, how many miles are on your car, any track time?

KC-CarGuy

Uh wait... did you just post in the turbo forum asking about hearing a whirring noise upon acceleration? Am I the only one that chuckled when reading the headline?