Ive got a loose heat shield just behind the engine bay and it makes a noise similar to the one described. When your sat in the driving seat it sounds like its coming from the left hand side wheel arch/engine bay. Ive crawled under and touched it with my hand and its definately loose. Problem is ive no way of safely working under it, as im sure it would take about 5 mins to resecure. Think i just gonna have to put up with it till service time, unless theres anybody local who's feeling charitable one evening/weekend!