Guitar picks up computer interference
Any type or thickness of steel will form a "Faraday Cage" shield to keep digital hash from escaping from the computer case.

HOWEVER. many cases have large holes in them for "windows" where you can look through and see the hardware (and the internal decorative lighting). I have no idea why this fascinates people, but who knows?

They also have large holes in the front for places where you can put extra floppy or optical drives, little panels with I/O connectors, etc. etc.  The front panels are made of plastic which offers ZERO shielding.

You could experiment with some aluminum foil from the kitchen and just cover the entire front of the case to see if that might be the interference leakage path. Be sure to connect the foil to the case ground at some point. An ungrounded shield is worse than no shield at all because it becomes a re-radiating transmitting antenna for the interference!