Possible to replace the fan for the NV+?

Can be replaced, but replacing the stock fan with a 3rd party fan voids the warranty. Using an incompatible fan can damage the hardware. If you still want to change the fan there are some suggestions on the forum. Do a search for "Noctua" and I think you should find some suggestions for the NV+.

Depending on the warranty you can probably get a replacement for the unit. I would call in to NETGEAR to find about your warranty and get a hardware check done.

A lot of people have replaced the stock fan with the Noctua NFB9 with good success
http://www.noctua.at/main.php?show=productview&products_id=16&lng=en&set=1

I just replaced mine and it's definitely quieter. I took a reading with my sound meter about 12 inches to the side and when it was booting up, I measured at least a 6-10 dB drop in noise. My temperatures are about the same, peaking at 40C for my hottest drive, and about 34C for the chassis, which is no worse than my old setup.

I just upgraded my NV+ to the Noctua NF-B9 on this forums recommondation, and im relly impressed about it's low noise.
I connected it with the black (low noise) adapter, and it's much quitter than the stock fan, you can hardly hear it at all.
The temp is around 40 C.
I will keep an eye on it to see how the temperature progresses.

Just came back and found it at 47 C...hmmm
It's located in a closet, so maybe it gets a little warmer than in general.

That is probably a good assumption. I have three desktop ReadyNAS units in this room and it is warmer than the rest of the house, but because of the ventilation not too much warmer. In a closed, unventilated space it could get rather warm. If you had it in a closet, the stock fan should not have bothered you.

Well a closet might not be the correct word to describe the place where im keeping it :)
The word to describe it is a "secretary" i think, but it's in the living room, next to my desk, so the stock fan was very loud and annoying.
This noctua is almost soundless in comparison.