Hi there.
Im trying to record something via my stereo mix but while nothings playing at all (no music programs open either) renoise shows and records noise.
Ive never had this before, could anyone please help me out what this could be?
Thanks 
This has nothing to do with renoise IMO, it is dependant on your sound card or PC performance.
Try to use other audio recording tools to monitor your stereo mix input and see if you’d notice the same amount of noise again.
Shit audio interface.
Are you using a generic internal sound card? If so, your problem is right there.
Any external interface of reasonable quality will be silent and improve your recordings a hell of a lot.
Alright!
Thanks, another great reason to save money for an audio card