My Peavey classic 30 combo rattles, and i THINK it's the speaker? Would a bad speaker be able to make a physical rattling noise? would changing speakers help?
Have you tried to tighten down the speaker screws just in case that IS your problem?
I think it may be the tubes. I think this happens to a lot of C30'S. You can buy these tube holders off ebay.
https://ssl.eurotubes.com/euro-i.htm?page=faq#13. scroll down and it should "there is a rattling noise from my combo amp"