I have the harmony clippers which I use on my nervous horse, yes the are quiet, but do have a good look at them before you buy, i find that the on and off button is in a silly place and is quite easily pressed in error. I have not yet used them for a full clip so far I have just took feathers off and hogged.
I found the cheapest place for them was Hope Valley Sadderly.
My horse was a nightmare to clip, so much so I ended up in a&e with a fractured skull a few years ago (note to self: always wear riding hat when clipping!).
With the harmony clippers, and taking it slowly, he is now very much better to clip. I have used them on a number of other 'difficult' horses and found they work brilliantly.
Friend has a set and they seem to work pretty well and fairly quiet... the bolts/pins got a bit rattly though if you didn't tighten them all up fairly regularly though...
I have the Lister Star clippers and would highly recommend them for a nervous/fidgety horse. They are lightweight and very quiet and i did two blanket clips last winter one after the other and they didnt overheat or anything. They were about 196quid on the internet.