Yes the PC is very quiet. During normal use, you cannot hear it. It is only when I am actually outputting video that all 4 processor cores come into play. The cooling fan spins up a little, but the noise level is still very low. I don't use an insulated case as the last time I did, found it didn't really make much difference.

I think the secret to a successful quiet PC is to use quality cooling products. It doesn't have to be expensive either. For example, the Arctic Cooling CPU cooler was £14.