Mesa Boogie Buzz

I have a Mesa Subway Rocket which is giving me some problems. When I turn it up to a decent volume, I get a buzz from the amp. I think I have determined that the buzz is coming from the amplifier, possibly from the speaker itself. I removed the speaker from the cab and played through it, and there was still a buzzy sound...So, I am guessing that it is in fact the speaker, but it really sounds like there is something rattling in the amp itself when I put it back in. Any suggestions???
thanks,
nate