> Start by replacing the caps. If the noise persist, move on to the resistors and
> diodes near the switching transistors.
> (assuming the noise wasn't coming from a fan)
The problem has not re-occurred with the new PSU even with four of the fans switched off, and just the two case fans blowing in, even though seeding and folding at maximum.
With working in the electronics business, it is not worth my time trying to repair the duff PSU. I could repair it but it is cheaper to just replace it.