| Go inside and snug up any+every bit of hardware you can get a wrench or screwdriver on. Don't forget jack plates, handles, corner hardware, and so on.
Pull the drivers out of the cab, swap their locations and rotate them, say, 90 degrees, but make sure the wires aren't touching the cones when you put them back. Tighten in a (progressive) star pattern, same as wheel lugs, so you don't warp the frames, which would cause the voice coils to rub. Don't sock the screws down, just snug them up.
You can also try temporarily shimming at different points where slight gaps exist, such as between head and case. That can help narrow things down.
It's possible, but unlikely, the cab has started to come apart from vibration, and that's the buzz you're hearing. You can try physically compressing the cab while you're playing, to see if the problem disappears. Try each of the 3 planes separately.
Also, if the cab is ported, could be the port shelf has loosened (if they use a shelf), or a port tube has come loose (if they use tubes). You can check that while the drivers are out.
Everything above assumes the speakers are NOT the cause. But if you go though all the above, even if you're 75% sure the speakers ARE the cause, it will bring your confidence level up almost to 100%.
Buzzing is one of the more frustrating symptoms to assign a proper cause to. | 
