Check this thread. It’s basically silent when used as ROCK.
I have a M1 Mac mini, 2017 27" 5K iMac and a 2014 13" MacBook Pro Retina so I can definitely compare them with the M90n but to me it’s just a waste of $$$ having a M1 Mac mini as Roon Server unless you use it for something else, which is not my case as I have already enough Mac at home. I just want a dedicated Roon Core, that’s all. If you plan to use the M1 Mac mini for other tasks, then that’s a no brainer.