It could be almost anything. What I`d do is play each part of the drumset individually: that`ll isolate the part which is rattling. You should be able to work it out by playing it (softly) while puting your head close to it. If it`s a loose screw, tighten it! If it`s something a bit more serious, you may need to take it to a drum shop. Hope this helps.
Thanks for the advice. I bought some control rings and I lined the bottom of the drum with towels. The set sounds great now and there is no more vibrating!
