@LoinKing what model NVR do you have. The modification in post #13 using two power supplies is easy enough. I wish I'd remembered that post before I took a 7616NI-K2/16P and through it in a skip because I couldn't locate a replacement PSU for one that was rattling...
I have NXI-K without POE, so just one case fan. I did see the posts about cutting off the metal grill to reduce noise, but some people said that didn't help them much. It might be best to start with a Noctua fan (running at low speed) to see how it goes.
Our nvr is connect to our main room TV - for a quick view of all the cams while watching tv, so not possible to move it elsewhere.