I once had a similar problem on an old Peavey bass head. The culprit was a loose rivet that mounted the transformer. Drilling out the rivets and using real screws, nuts and lock washers solved the problem.
You SHOULD be using blue Loc-Tite on the speaker mounting screws on any bass cab you own. If this was not done at the factory, then it should be the first thing you do to a new bass cab or combo.
On my recently upgraded Marshall AVT50, when I replaced the crummy stock Celestion with a Celestion G12H-80, the original lock washers were INSIDE the speaker basket mounting holes and were serving no purpose. Typical English electronic product where a basic flaw is repeated over and over and any nitwit could have seen the lockwasher and hole diameter were the same size. Threw away the lockwashers and used blue Loc-Tite.
I noticed that the AVT transformers are also rivet mounted, but since I am selling the AVT50 I left it as-is.
Tubes are also notorious for becoming microphonic at a particular frequency and may sound just fine until you play the "wolf note". May NOT buzz when you tap it, as you are ONLY exciting the tube self resonance, and not the wolf note resonance frequency. No tube tester checks for microphonics, it can only be tested in the circuit under actual use.
A signal generator set to the frequency that causes buzzing will help you quickly locate the cause of the buzz. If you don't have a sig-gen, then plug in a microphone and sing at the particular note frequency, or have somebody fret the note on the instrument and use high gain to sustain the note. Easy to rig a guitar with some tape and foam rubber under the strings to mute unplayed notes as the amp sustains the problem note. You can probe away in the amp and cab to your heart's content until the buzz is found.
My biggest problem is objects in the room that buzz, like cabinet doors. If you live in a home with sheetrock or paneled walls, then you already have a very low frequency resonance problem that will cause a peak and dip in your bass response that may not be perceived as a "buzz", and large cardboard boxes cause a similar problem at their box wall resonant frequency. Washing machines, clothes dryers, refrigerators, freezer chests, toaster ovens and unoccupied metal chairs can all have Hi-Q resonant panels that ring and buzz. A door knob with loose internal parts is one of many things that can drive you crazy when playing bass in a house.
