| Pull the speaker to see if the wires from the amp are resting on the top of the cone (long shot I admit). Check that the spade lugs have a good grip on the speaker terminals. If they do, then that's not the problem, so pull them off and put the speaker aside. Pull the head out of the cab and poke around inside. (Power off, AC cable unplugged.) Make sure everything's still tight. Find every removable cable, connector, etc., and reseat each one several times to get rid of any oxidation. Check the screws that hold the circuits board(s) to the chassis because oftentimes they're used to establish earth ground so if one comes loose you can get static-y problems like this. Check anything else that might come loose from vibration. In my experience, vibration is the most common combo-killer because it tends to cause screws to back out, circuit board traces to crack, solder joints to pop, etc.
That's where I'd start. | 
