beatlessteve

Hi, I recently bought a line 6 spider 3 half stack, and while the amp sounds terrific, when i turn the amp up around 4 the bass strings on my guitar create excessive vibration at the back cover, and the cover seems to be on tight...I havent yet taken the cover off to check the conditions of the speakers. Its just a loud vibration and sound like the back cover is vibrating real loud. Has anyone else had a similar problem? Thanks, Beatles Steve