Whacking is a valid technique, I was not joking. If your fist isn;t handy, use a rubber mallet.
I was assuming the little chassis was bolted into the little cab, but you can whack the end of the metal chassis, hell use a heavy screwdriver handle, or really even a metal hammer. We want to send a mechanical shock through the thing, that will expose loose/cracked connections or even parts with internal issues. If we find anything is sensitive to being hit, we then know to look closer for the individual thing causing it.
Larger parts on the chassis sometimes vibrate enough to crack their solder, then the vibration from sound can cause momentary loss of connection. Larger parts are the main filter caps, larger power resistors, and also things along the panel like jacks and controls. They can crack their solder.
Poking on the board is best done with an insulated probe, which is a fancy way of saying a wooden chopstick, like from Chinese restaurant.
