What you're describing is a rattle, likely from something contacting something else. Butyl deadener will help, but something like CCF would help just as much if not more. It's hard to say until the piece or location is tied down. If it's a resonance issue from volume, then yeah..deadener will be a good start. If you're looking for something cheap then try Raamat. It's decent enough to get the job done, while still using quality materials at a good price.
I've got a few sheets of CLD tiles (from SoundDeadenerShowdown) and CCF (also from SDS). I could ship them to you if you're wanting to try some stuff. It's not cheap stuff, but it's the best there is in my opinion.