Tranny failure

[Murphyv310]

That gives me a big clue as to what is going on.
It will be a duff decoupling capacitor that has gone open circuit allowing the amplifier to become an AM transmitter.
One big long wave transmitter. Check it out with a long wave radio and you should be able to tune into it from the end of the street.

Ha Ha.
That could be fun, with an audio input I'd expect the oscillation to vary so it could be a Long Wave FM transmitter

Could even be parasitic oscillation.