I had the EXACT same problem a month ago. I had to replace the power supply in
my computer, and the noise was horrible.
Normal noise was at about s4, and when I turned on the computer, the noise
level went up to about +20db on the signal strength meter. The only solution
that worked for me was replacing the power supply. I tried antenna changes,
ferrite chokes, and grounding the Computer cabinet.....none of that helped.
Good luck, and hope you find a power supply that works out.