Mcintosh MC431M in power guard

I have a Mcintosh MC431M, when it is turned on, there is a loud noise in the speakers before the power guard light goes on.
I have removed it from the car and connected it to a power supply without speakers, same thing happens, it goes into power guard mode.
I have opened it up so I can see the circuit boards and components, there are no visible caps leaking or anything.
The problem seems similar to
But there was no solution posted. I only know basic electronics, and I can't find anyone to help with the repair in my area. Does anyone have any clues where to start?