What's this noise?

Post by: **Richard J** » Tue Nov 06, 2018 5:14 pm

Looking for a bit of help...I'm getting a rattling sort of noise on the overrun in my +2 fourspeed which I've recorded on my phone and attached.

Sounds like it's coming from the bell housing area, or behind the dash somewhere. I only get it when the box is warm, and when I lift off the throttle at at 40 - 50mph. I'm not much of an expert on gearboxes, but I'm guessing idler shaft, or perhaps even the release bearing vibrating - but they are very uneducated guesses.

I'd love it to be the ashtray or exhaust mount but I'm pretty sure it's internal. Please listen and let me know if you've heard this before.

Couldn't attach the file (not allowed) so here'a link

https://1drv.ms/u/s!AsOq-jbOtbQkguJkUVjpVStsIMMmhA

Thanks all

Richard

Post by: **david.g.chapman** » Tue Nov 06, 2018 6:51 pm

I am not sure about the idler shaft (others may know better) but the release bearing could be a candidate. Try riding the clutch to take up the free play in the release bearing when you are out next on overrun. This should eliminate it.

The whole drive train shifts when on overrun. Other things to look at are the torque rods on the diff and the diff mountings. The diff should not be "spongey" but well held in position. Check everything else that might contact the drive train or exhaust - within 5mm? Also check nuts for tightness everywhere!

Dave Chapman.

Post by: **Richard J** » Wed Nov 07, 2018 1:48 pm

Dave, Thanks for the reply.

I'm pretty sure it's at the front end as the diff is pretty solid and I've had a good look at the exhaust mounts and torsion bars and bushes (when tracking down other noises!) .

I'll have another go with touching the clutch to put a bit of pressure on the release bearing to see if that helps isolate the problem. However it's trickier than it would seem as the noise stops anyway as soon as the clutch starts to disengage the gearbox and so far when I've tried it traffic conditions haven't been conducive. I need to head out onto a dual carriageway when the traffic's quieter to be able to experiment a bit more.

Any other suggestions welcome!

Richard

Post by: **types26/36** » Wed Nov 07, 2018 2:14 pm

The noise does sound like a gearbox type noise but recordings over the internet are very deceiving, is the noise only in fourth gear or is it still there in third gear? just thinking that forth gear is a direct drive through the gearbox and there is no load on the layshaft but the layshaft bearings are loaded in third gear.

Post by: **Richard J** » Wed Nov 07, 2018 3:19 pm

Brian - excellent point! I don't think I get it in any other gear than fourth, but I will go out (when it stops raining!) and see if it will do it in third as well.

Another theory is synchro ring rattle, does it sound like that to anyone..? Anyway I'll report again when I've tested for release bearing and layshaft noise .

Incidentally, I've listened to the recording via the link - and it does sound pretty much like it does from inside the car.

Thanks all,

Richard

Post by: **Richard J** » Thu Nov 08, 2018 11:36 am

I've had run down the A3, and I only get the noise on the over run at just above 2000rpm, if I touch the clutch the noise doesn't stop, (until the clutch is sufficiently disengaged for the load on the gearbox to reduce) and it doesn't do it in third., (running at the same rpm).

So all that leads me to thinking that it's the layshaft. As it only happens at around 2000rpm on the overrun it doesn't feel like I'm heading for an immediate failure, but would appreciate others' views on this.

It feels like something is resonating at that particular engine speed - the sound has an oscillating quality, the pitch rises and falls slightly...

Anyway my questions are:

Does the layshaft seem like a sensible diagnosis - if so what actually might be wrong with it?

and

It's running on bog standard GL4 EP90, could something like Redline synthetic make any difference?

Thanks all, Richard

Sound file here: https://1drv.ms/u/s!AsOq-jbOtbQkguJkUVjpVStsIMMmhA

Post by: **jimj** » Thu Nov 08, 2018 12:43 pm

Try the same road speed in 3rd as 2000rpm in 4th. Everything back from the gearbox will be rotating the same and if you don`t hear the noise you know it`s the gearbox or clutch. If you do hear it then you know it`s somewhere from the propshaft backwards, which it may well be.
Jim

Post by: **Richard J** » Thu Nov 08, 2018 2:29 pm

Hi Jim,
Neither in third with the same road speed do I get the noise, and nor in third with the same engine speed - so I think that means the noise is coming from the gearbox, also if I dip the clutch it stops.

So as Brian pointed out 4th is direct drive, so no force goes through the layshaft, allowing it and the gears and synchros on it to rotate and vibrate freely. That suggests to me that it could be a layshaft issue, although I'm hoping that somebody that really understands the 3 rail box could confirm or deny it!

Richard

Post by: **Craven** » Thu Nov 08, 2018 2:52 pm

Just to put the terminology right, the lay shaft is a simple parallel shaft on which the lay gear runs on 2 sets of needle rollers. Lay gear is a single part having the gears machined onto it, one piece no separate gears or synchro?s on this gear.
FWIW

Post by: **Rich135** » Thu Nov 08, 2018 3:59 pm

Hi Richard

Sorry to hear you are having issues. I am by far the best person to advise on the causes of it, but am happy to jump in with you if you need a second pair of ears to feel/hear around while you are driving.

I had a very annoying rattle for the past few months which only happened when moving (couldn't replicated it when stationary!). It turned out it was just a loose screw in the nearside rain gutter rail!

I hope yours is such a simple fix. Around tomorrow if that helps.

Cheers

Rich