Hiss from studio monitors with Mic Connected to PC

Thank you for allowing me to join the forum, and I hope that my question is not misplaced.

I have a nice Hi Fi set up for my living room and I wanted to ask you about my office set-up. I play Spotify via a pair of Rokit 5 studio monitors, via a Nano Patch attenuator and a Mix5 deck, plugged into an Acer Aspire windows 10 desktop, via the rear input. It's a long story...but it was originally my set-up to play an EWI with backing tracks from the computer, mixed down with the Mix5 and Nano patch.

My question is a fairly simple one. When I connect a microphone (a Rode Stereo Pro) into the mic input next to the speaker input at the back of the desktop, I get a hissing sound from the speakers. This is with the mic on or off (and it's usually off). It gets louder if I turn up the attenuator.  It goes away if I unplug the mic. Can anyone suggest how it might be possible to leave the mic plugged in, and not hear the hiss?

Thanks for any replies and I hope, again, that I've not hijacked the forum!