The fluctuation isn't necessarily a big concern. Open Task Manager or another monitoring program to see what your CPU is doing. While you may believe it is simply idling, you could have background tasks spiking CPU usage - which is likely, and would cause those fluctuations. The temps you are quoting look about normal for a 3770K @ 4.5 with an H100i.
Grinding noises are never good things coming from a pump. The H100i doesn't have the most stellar of track records with pump issues, either, so I wouldn't ignore it. I would definitely recommend exchanging for a new unit (or better yet, a Swiftech H220-X ). No matter how much of a PITA it was to put in, it will be more of one if the pump quits on you 21 days after purchase.