I have a Roland Cube-30x and a few days ago it started making a buzzing/rattling noise, and it wasn't quiet. I started worrying and, to exclude possibly more serious problems, plugged-in the headphones to see if it was the speaker. I was right.

So, what should I do? It seems related to feedback, and some notes seem to cause more rattling than others. For example, if I pick and hold E 10th fret 4th string, when on mid/high-gain, it goes on and on rattling loudly and making feedback noise.

The only reason I find for this is that, the day before it started happening, I went jamming at my friend's, but I didn't bang it anywhere while carrying it. Not that I remember, at least.

I'm really worried. What's going on?
might be a lose connection
A hero of war, Yeah that's what I'll be

you might just be rattling the amp at it's resonant frequency.. like i know that if i play 2nd fret on my sixth fret, my garage door rattle's like crazy... i think this might be happening in your amp.. it might not be your speaker, but rather that the wood of it is rattling... try and see if anything in the amp is loose, like the handle on top, or something like that.. i hope this helps
Thanks for the replies. I don't think it's a part of the amp that's causing this, FightinIrish. Imagine the sound you get when blowing air into a mic. I'm not sure if that kind of noise could be generated by a loose part...