@bojack Don't panic! This is solvable.
The first thing to do in a situation like this is move the monoblock to the other channel and see if it does it there. Apparently the other monoblock is fine, right?
If its quiet in the other location, then its a pretty good bet that something about this amp is more sensitive than others to noise that is present at the first location, like a digital device such as a wifi router, DAC and so on.
In any event this sounds like an RFI issue, which might also be caused by an oscillation in the amplifier itself. It might be just on the edge of oscillation and so behaves in other locations. Hypothesis of course.
So if the amp still makes the noise in the other channel, then what you do is remove the input tube (while the amp is off of course). It should not damage the amp to run it without this tube. The question is, is the noise still there? If not then we have a very good idea of what circuit is causing the noise!
If the noise is still there, then the 2nd (driver) tube can be removed. The driver is capacitor coupled to the output section so this can be done without damage, but again the amp must be off when this is done.
One possible way the amp can make this noise is if a certain part was omitted, called a 'stopping resistor'. It sits (or should) at the input of each tube in the amp. If not there, the tube can go into oscillation. Often people get away with not installing these resistors in their DIY project, but they are good practice. But if the amp is handwired, its the sort of thing that would be easy to forget on accident and might be really tricky to figure out later since the amp might behave, although not all the time.
Try these tests and get back to us.