I have developed a new rattle / knocking noise on my mk3 tdci.

Car ticks over fine at 800 rpm, and no noise, other then normal diesel engine noise.

When I pull away, it has a loud rattle knocking noise but goes when I hit 1000 rpm and engine is quite. It sound like its coming from the alternator side, defo not gearbox side.
Its the same in each gear. It gets better after about 15min drive. Had new pulleys 2 year ago.

Any ideas? Only really noticed it after my new exhaust was fitted. Might be just a coincendance? Don't think its a heat shield as don't sound tinny.

Cheers