I have encountered similar issues with the Marshall Class 5 Combo and with other amps so ill offer some advice on what to look for although none of this is amp specific.
Some modern amps use heat sinks (like the ones in your computer) some times wires can rub up on them and if the wires have a zip tie around them they will vibrate against the sink. Since the heatsink is kind of like a tuning fork the sound can be somewhat audible and cause a buzz like that.
Likewise the PCB or other components in the amp are often bolted down and use plastic grommets some times these wear out (if you move/bump the amp a lot) and the whole PCB moves around causing a buzz.
The amp most likely contains a transformer which are sometimes shielded in a stamp metal box, again held down with screws this could be rattling around making the buzzing.
The speaker cone could have some damage causing it to not be vibrating as it should or rubbing against an internal component.
I dont think this is a tube amp but if it is some amps have thin metal clasps that hold the tubes in that can rattle agains the side of the tube.
Generally tightening things can some times solve the problem. Be careful about adding things like tape to the internals of an amp as they often heat up and can catch fire if you don't use things made for this.