Thanks everyone - some good ideas there to be working on. I had eliminated the wishbone/crossmember possibility. There was a slight knocking there as well, but I have just fixed that and replaced all the bushes and checked the chassis bolts. Gearlever carrier - yes, been there some time ago. Cured with new engine mounts, and centralising the gearbox mounting by altering the mounting hole slightly. Cat - wrong side, nothing external rattling, makes no noise on jiggling or hitting it. Baffle in rear silencer. A possibility, but a bit far back. That's my nearly new stainless steel one from the Club supplier. I think, but can't be sure, that the noise was there before. However I can't make it rattle by jiggling the exhaust. Something in the left hand cill is a possibility - it's the right location for the apparent sound. It's hard to picture what could rattle on small bumps but be silent on big, and why it's worse when cold when the Waxoyl is presumably stickiest. But I'll take the seat belt mounting off and have a look. Failing that I suppose I could try temporarily unbolting the shock absorber. Oh yes - not wheel bearings either.