I play through an 80W 2x12 combo, and I get LOTS of rattles. I think its kind of inherent in a higher-wattage combo because the chassis takes so much abuse due to all of the vibrations caused by 2 12" speakers going crazy at high volumes.
It could be a few things, speakers included. The classic 50 is a tube amp, so it could be one or more tubes have become microphonic and are amplifying the vibrations of the chassis, or there could be something other than a speaker loose inside the amp (I had to do lots of tightening on my amp right after I got it to quelch some rattles), or it could very well be the speaker itself. Heck, my room rattles more than my amp at this point...there are lots of knick-knacks and junk laying around in my "studio space" (ok so its just an office-like room with no acoustical treatments whatsoever). Sometimes I have a hard time telling whether its my room or my amp that's rattling.
I'm sure that there are a couple of other causes that I'm not thinking of, but its not necessarily the speaker that causes these mysterious rattles in a combo amp.