Sideways, the way forward

Had my oil changed the other day and when i got it back after a swift blast to feel the smoothness of its new lubricant, i noticed (when parking) a slight rattle coming from what sounded like the exhaust... It can only be heard when blipping the accelerator whilst standing still with the window down. Sounds like it is coming from under my seat almost.

I took it back to my man who changed the oil and naturally it stopped doing it. It was hard to fathom where it was coming from seeing as it had temporarily ceased to be a problem. Wonderful!

It only seems to do it when the engine/exhaust is hot...i.e. after a blast.

Doesn't sound like anything too serious i just wondered if anyone had come across anything like this.

It could be a heat shield i suppose but something tells me its not.

any ideas???