Loud, at least the ones I've been around. I'd say they're quite a bit louder than a regular hair dryer on the highest setting.
Forced air dryers and a good pair of straight shears are my favorite things. Nails are my least favorite thing to do, but I do like a nice super-sharp pair of clippers and a dremel with a broad sand barrel for taking care of nails. Right now I have neither and it sucks.