Ashdown mag300 loud buzz and screeching

[FunkeeFresh]

Was about to start recording bass for my band's demo and my ashdown starts buzzing really loud and screeching. The amp has worked fine for years, I checked the boards inside and there's no evidence of damaged components or cracked solder. I emailed ashdown and they said it could be an earthing problem but the earth just connects straight to the amp chassis from the kettle socket.

Anyone have any ideas or experience with this sort of problem??

It starts buzzing as soon as the amp is turned on even with no input. The speaker is working fine as I tested it with another amp