I found our previous PC was putting a lot of noise onto the mains lead and it was radiating all over the house.
So I made a short flylead which plugged into the PC's IEC socket and was fitted with a filtered IEC socket, see picture. This socket was mounted in a small plastic box for safety. Once fitted, peace & quiet reigned again!
The filtered socket came from Surplectronics.
https://www.surplectronics.com/ It's listed under capacitors.