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[Bojan]

Hi!

I have a constant problem with a strange knocking noise coming from the exhaust manifold/downpipe area. It is like this:
 - knocking/hammering sound
 - it comes from the exhaust manifold or down-pipe area (left bank)
 - it appears only under harder load (accelerating moderately or hard)
 - it does NOT appear at all when accelerating slightly, or when reving it (no matter how hard) in neutral
 - no nosie can not be heard from the top of the engine, or from under the car

I was under impression that the heatshield on the downpipe was rattling, and it was, so I removed it yesterday. This leveld the noise consideralby, but the knocking sound can still be heard (however, it is less noisier now, different, and appears only from 1500-2500 revs).

Any advice on curring this mistery noise? It's driving me nuts.
It's hard to detect it, because you can hear it only during the drive.

BTW, it's 1996 3.0 MV6.

[78bex]

Not sure if same as yours, but I had a similar annoying knock @ speed on an astra once & after taking all apart it was the catalyst. I had ignored an error code light on the dash for ages which finally over heated & destroyed the cat.

[Doctor Gollum]

N/S engine mount working loose

Check both nuts, top is 16mm, lower is 17mm.

Lefthand one always fails as the engine rock to the right when you rev it. Booting it left off a roundabout rocks it enough to allow the mount to fall out

[Bojan]

I've had it checked by several mechanics and exhaus guys, and they all claim that there is absolutely no leak in the exhaust system.
However, i have a replacement exhaust system, with no cat (pipe is welded in its place).
I was warned that this could be causing this sound.

[Bojan]

Finally, this is solved 
As it turns out, the problem was in de-cat pipes (just a pieces of pipes welded in the place of cats).
The cats act as a mufflers, and removing them, I had a long pipe with two 90deg bends, trough which un-muffled exhaust gases ran, causing a 'knocking' sound under acceleration.
It was cured by installing two absorbtion mufflers in place of former cats.

To conclude: if you are removing cats, place a muffler on it's place to prevent knocking.