Funny noise on a TD5

I finally managed to fit a de-cat down pipe to my disco at the weekend and everything was fine to start with, but it started to make noise like it's vibrating against something but only at 1500rpm. I've been underneath and there's nothing that can touch the pipe.

I though it might be just that it's a new pipe but it' still there and not going away and I done about 200 miles since fitting.

Has anyone had this before and how do I solve it.

sounds like it's most probably the heat shield above the exhaust rattling.

Had a look the other day but couldn't see anything and the pipe was hot, now is raining so there's no way I'm laying under it just now.
I'll try workshop later and see what I can find.