Gransport whine & hiss

[Oxytorch]

My GS has developed a mysterious noise.
Like all these gremlins, it's a bit intermittent.
It seems to only occur after cold start but not right away and it doesn't happen every time although it's becoming more frequent.
After driving off about 30 m, a combination of a whine and hiss suddenly begins together. There's no noise to begin with.
The whine is most noticeable when driving at low speed and it's pitch varies with the speed of the car, not engine speed.
The whine persist if coasting in neutral.
Both the hiss and whine gradually fade within a couple of minutes of driving.
Once warmed up, the hiss can only be heard when applying some throttle & can only be discerned with the window down driving along a solid barrier that reflects the sound.
Ease of the throttle and the hiss goes. The his is either on or off; it doesn't vary at all in intensity with the amount of throttle applied.
The hiss had been occuring for a while before the whine developed. I had thought the hiss was an exhaust leak, but now I'm thinking it's a friction noise because it begins at exactly the same instant as the whine noise starts.
I'm not sure if has any bearing on it but the car also has had a clutch howl for over 6 months that only occurs briefly when stomping on it from standstill or steeper hill starts but it hasn't gotten any worse.

I'm going to book it in with the only Indie here in Brisbane to get it checked out but any thoughts or advice would be appreciated.

Cheers.
John

 

[CatmanV2]

Air leak perhaps, but that would give performance issues I'd think. I wonder if it's the aux air pump.

C

 

[conaero]

I would look at the secondary air system and Aux belt. It may be your AC pump is on its way out. There is a pipe for the secondary air that goes round the front of the engine that rusts but hissing is not normally an indication.

Take the aux belt off and start it and see if it goes away.

Press the ECON button that disengages the AC pump and see if that makes any difference.

 

[Oxytorch]

AC compressor was replaced a year ago when it seized up. Scary belt squeal & smoke!
Have tried ECON - it makes no difference.
Having said that, the new AC compressor which cost a small fortune has sounded like a small washing machine ever since it was replaced, the noise waxing and waning periodically but only audible inside the cabin.
The AC noise does stop as soon as ECON is pressed. That's another thing that needs looking into. I read somewhere that this could be due to incorrect amount of refrigerant.

 

[CatmanV2]

Possibly. Mine was similar in the 4200, but gas level was OK

C

 

[conaero]

It may just be a noisy aux belt, take if off and see if it stops, its only a single bolt to lever the tension. Just make sure you make a note of the belt fitment, you won't be the first an needless to say the last to come unstuck with this

 

[Oxytorch]

Hmmm. Correct me if I'm wrong but the whine/hiss only occurs when the car is in motion, so not likely to be the Aux belt? Neither noise is evident when stationary whether in gear or N. The whine is superimposed on the normal gearbox sound. The pitch of the whine varies with car speed. It gets lower as I slow down and come to a stop, at which point the noise is absent. This is the case if in gear or N but the pitch of the whine does blip up when shifting down gears. My guess is some sort of gearbox/diff noise but it's peculiar in that it occurs in gear or N, and it gradually fades away although it is tending to take longer to become totally quiet - several miles.
Maybe gearbox oil is dirty or low?

 

[BL330]

John, I once had some sh1tty tyres on a BMW.
The howl which I was sure was the gearbox on the way out, was in fact those tyres.
Just a thought.

 

[CatmanV2]

It could, I guess, be diff whine, but doesn't really sound like it to me.

C

 

[conaero]

Does the sound change in pitch the faster you go or with revs?

 

[boomerang]

Maybe (one of) the bearings inside the torque tube is/are getting noisy.

 

[Oxytorch]

The whine pitch seems to vary according to car speed. Over about 3000 rpm it becomes difficult to hear over the exhaust noise. While coasting at say 20 mph, if I drop into N, the whine doesn't change even though rpm drops below 1000 rpm. However, the pitch gradually drops as I slow to a stop. There is no noise when stationary and reving in N while stationary causes no noise.

If torque tube bearing was at fault, wouldn't it make noise even when stationary & in gear??

 

[adam01]

The whine pitch seems to vary according to car speed. Over about 3000 rpm it becomes difficult to hear over the exhaust noise. While coasting at say 20 mph, if I drop into N, the whine doesn't change even though rpm drops below 1000 rpm. However, the pitch gradually drops as I slow to a stop. There is no noise when stationary and reving in N while stationary causes no noise.

If torque tube bearing was at fault, wouldn't it make noise even when stationary & in gear??

Suspect TT bearing noise will be present when in neutral, typically if left in idle for an extended time

 

[Oxytorch]

Indeed my cars TT does that if left idling in neutral for several minutes but it's a sort of rattle that disappears as soon as 1st is selected. I didn't think that was a big issue. It sounds totally different to the noise when driving now but maybe it is related... I don't get why the noise goes away after driving a while. I guess there's some expansion wherever it's coming from which causes it to quieten down.

 

[conaero]

Indeed my cars TT does that if left idling in neutral for several minutes but it's a sort of rattle that disappears as soon as 1st is selected. I didn't think that was a big issue. It sounds totally different to the noise when driving now but maybe it is related... I don't get why the noise goes away after driving a while. I guess there's some expansion wherever it's coming from which causes it to quieten down.

Thats normal.

 

[Oxytorch]

Ok. I'm pretty sure it's some sort of transmission problem now. Driving it home this evening, the hissing noise seemed louder. When I pulled up on the driveway, the loud hiss continued even when stationary and in neutral. Previously, the noise stopped whenever stationary. Then when trying to select 1st it made repeated clunking noises and refused to select a gear. The hissing sound stopped each time while it was trying to select a gear. Finally it selected 2nd and I was able to ease the car into the garage.

 

[CatmanV2]

Hmm. Sounds like a dealer then

C

 

[safrane]

Could the hiss be a light grinding?...possibly the thrust bearing hence the gear select fault.

 

[conaero]

Looking like a bearing in the transmission.

 

[MAF260]

Clutch issue. Check for leaking fluid under the car, could be the hiss of hydraulic fluid from a pinhole leak. The clunking is not a good sign, nor the 2nd gear selection. There's a very good chance you won't be able to select reverse when you try to next move it from the garage. Time to raid the war chest as I have just done!