Cabinet resonance problems with the great little amps is a major nuisance. The cause is the chipboard baffle. This would have been ply in the old days but in Fenders reissue they have gone for a more affordable design choice.
You may be able to fix yours just by tightening the securing screws up a little. It’s a hard life being a combo and they will loosen off in time.
See below for an attempt to solve the problem with a little DIY cabinet rework.
Remove baffle add bracing at top. Some folks like bills used angle aluminium but this model already has had a ply brace added but it is just pinned in. Here I have added several decent screws carefully drilled in, glue and some additional brace.
Refit baffle applying wood glue and adding additional securing screws.
At the back of the amp I added a brace to reduce the rear panel resonating.