I had a particularly odd, quite harsh metallic rattling noise above 3000 revs which mysteriously disappeared after I had the timing belt done. At first I thought it was a loose exhaust baffle but I never figured out what it was. This rattle would appear in 500 rev chunks. From 2800 to 3400 and from 4400 to 4800 but not the whole way through. Was very very strange! Can only assume a helpful mechanic spotted what was wrong and fixed it without mentioning it.