Marshall MHZ40C - Buzzing

[Pbateman]

I recently bought a MHZ40C online from eBay. It got in yesterday and is buzzing loudly. This is not a feedback buzz, rather this is the actual amp itself rattling. It is especially bad when I play low pitch notes. Any thought on how I can fix this. Thanks

[Jazz P Bass]

Marshall MHz 40

Sounds like a mechanical rattle to me.
All that I can suggest is for you to take it apart & inspect it.
Then put it back together better.
Of coarse you have to rule out the speaker.