It all depends on the sump and the stand and the drainage.
If you have a wet dry in a non sound proofed stand with only a standpipe then it will sound like a cross between a plughole and Victoria Falls.
However, if you have the end of the standpipe submerged, on a decently made stand and a durso sandpipe it can run silently.
I have a wet dry on my big tank and the noise of the water returning from the pipes is louder than the drainage (which just sounds like a little indoor water feature).
I haven't noticed a huge amount of evaporation just from the sump, but if it concerns you then put a drip tray over it.