If the amp has an effects loop, connect effects out to effects in with an instrument cable and see if the problem goes away.
Make sure the speaker is snugged up to the baffle board, i.e. no loose screws.
Make sure the screws holding the head in place are snugged up properly.
Check that the screws holding the handle to the case are tight.
Put something heavy like a cinderblock on top of the amp. If the problem goes away, could be the cab is coming apart.
Speaker could be blown, but there's no 100% reliable way to verify that other than disconnecting the internal speaker and connecting an external cab (of the same or higher impedance). You could, though, push on the cone and see if you hear the voice coil scraping.
VERY slight possibility the wire from the amp to the speaker is touching the back of the cone. I say very slight because why would it show up after 4 years.
Could have a loose spade lug on the wire that connects to the speaker.
One of the wires running from the spade lugs to the speaker's voice could could be shot and about to totally break off.
Maybe a cold solder joint somewhere in the amp. Pull the amp out of the box, set it on a pillow, and run a wire to the speaker. If the problem goes away, then you have to run down where that flaky connection is and re-solder it. That would be tech time. There's a possibility, though, it's just a loose screw or nut somewhere inside the amp, especially if it's a ground point. So, check to make sure all screws/bolts/nuts are snug. Also unplug and re-plug all removable connectors to scrape off any oxidation.