mysterious buzzing only with computer input

I have an old Yamaha MT8X hooked up to a recording computer with high-end sound card using the monitor out rca plugs. The sound is crystal clear and the noise is a very quiet static, perfectly acceptable level. Everything works perfectly UNTIL I plug in my other computer (the music playing computer) to the board using the 2LR rca input jack. Suddenly there's buzzing in the computer input. Here's what's so strange- I plug my headphones directly into the mixer, and there's no buzzing whatsoever. Furthermore, when I power down the mixer completely, the buzzing can still be heard in the computer recording, until the music playing computer is unplugged from the 2LR input. I tried the Aux return inputs using a different configuration with the exact same results- perfect clear music in the mixer's headphones monitor, but buzzing on the computer. Any advice would be greatly appreciated.

Did you try isolating the power supplies.

the computers are on separate surge protectors. I made some progress by using more shielded cables. I'm thinking if I get a good set of well shielding cables to replace everything in the setup, I'll be good to go.

I think PD means for you to plug it into its own power plug, with no other appliances connected (as in, its own power line from the fuse box). You'll probably find that power plugs in the same room are on the same line.

Just to test it, if you have an extension cord, in most houses the oven is on its own isolated line, so you could try that. If it fixes the problem then your issue is crappy electrics in your house, and you could either put up with it, or get an electrician in. Its a bit of trial and error really, unless you have the wiring diagrams for your house.

In most houses the oven is either hardwired in to the wall, or has a 30 amp plug, which is almost twice the size of the conventional 15amp wall plug.

sigh... CBF

Do go on. Please.