Noise (engine?) on over run

[OldRic]

I have a MK1 cab which I has TSR 2litre engine fitted last year,
I had the gearbox rebuilt about a month ago but am now getting a vibrating noise
on the engine overrun. I've had a look arround the engine and underneath the
car and can't see anything loose. A mechanic has had a listen and thinks it may be the main bearings? The noise is worse when the engine is cold. If I press the clutch in on the overrun the
noise goes away, however the noise does appear if the engine is reved with the clutch pressed in.

[EsSeX dAn]

does the noise always go away when the clutch is pressed?if so its probably the clutch release bearing.if it was main bearings youd know about it,its very loud and doesnt go away also the engine might be down on power

[OldRic]

Thankx for the prompt reply.

Only had the gearbox refurb about 4 weeks ago. The noise isn't as bad once the engine is warmed up.
On the way home from work today, when I was on the overrun and it was making the noise I pressed the clutch in, the revs dropped, the noise went. With the clutch still pressed in I bought the revs back to to about 4thou and the noise came back, but not as bad as when under load. I did this will the car was still rolling so I'm sure the noise isn't down to anything on the drive shafts/wheels etc.

Also the engine seems a little lumpy on idle but runs fine if I pick the revs up a tiny bit.

[OldRic]

Had it on a ramp today and with engine running and went throught the gears and there was no
noise but the wheels were off the ground, guy who looked at it reconds its to do with the drive train.
He suspects the diff, will have to take it back to the people who refurb the gearbox about 4 week ago.

[Wayne]

If it did not do it before the gearbox rebuild I would guess the problem is in a gearbox bearing or preload issue.

[OldRic]

I was told that if the oil pressure is low that it could be the main bearings going and that could be why I'm getting the noise when the engine is under load. So I had it oil pressure checked today which started at about 70psi but once warmed up it dropped to about 15psi on idle. Manual says to test at 2000rpm and it should be 29psi, when tested it was just about reaching 26psi. I've been told that because its not far off it not likely to be the main bearings. It going back to the gearbox refurb people to get a slight oil leak from the diff sorted and I'll get them to check the gearbox/diff for any problems.

[Wayne]

Might not be your main bearings, could be your oil pump

[danny_p]

an eas cheep check is to put a new oil filter on and cut your old one open,  look for flecks of metal   that soon tell you if it's thinking about going pop

[OldRic]

After adjusting the clutch cable the noise has gone away.
The after market ones don't seem as good as OEM, but VW don't do them anymore.