Engine 3.3 in Prime 4 broke in mid-session

Well that doesn’t make any sense. If you turned off the Prime 4 and the noise continued, I would say the noise is not coming from the Prime 4.

I think he means when he turned it back on after the power cycle the noise persisted.

obviously when the prime 4 was switched off it stopped making noise, but when I switched it back on the noise returned, until I disconnected the IEC connector and switched it back on.

Not obvious, because of the way you wrote it.

Anyway, this suggests to me that (as mentioned earlier) it’s some kind of power disturbance/glitch, as the power switch on the Prime 4 is “soft” and doesn’t stop power getting to the PSU electronics. When you disconnected the cable then the power was properly removed.

And how are you using the prime 4 to run music when all this is happening ? Some DJ software or standalone ?

standalone mode