Ok so I put the car over the pit and checked the exhaust... no visible damage, well bolted to the manifold too. My old man was helping me, and he discovered a really faint rattle in a section of the exhaust, about a foot or two from the manifold. I think it might be the cat converter cos its slightly larger than the pipework. We gave it a bit of a shake, then poked around a bit more and eventually found nothing more but when we left - no more noise. The system is an aftermarket Manta, installed by a previous owner. So now Im thinking a damaged cat. Worth pursuing, or is it best just left alone??