Werther makes very high quality compressors. I believe they are manufactured in Italy and they manufacture all of the compressors for Silentaire which is the most popular silent compressor in the world.
The cheapest Silentaire, the Eco 20 runs $389 from Dixie Art
I just found the info on that Werther here:
http://www.compressorsolutions.co.uk/wertherthree.html
It is 54 dB which is fairly quiet, but I wouldn't call it a "silent compressor."
By comparison a normal conversation is about 60 dB and a whisper is about 15 dB.
In comparison here are some dB sound levels of other compressors:
Iwata PowerJet - 55 dB
Badger Billion-Air - 30 dB
Silentaire Eco 20 - 30 dB
So that Werther is about the same volume as the Iwata PowerJet which is a popular illustrators compressor.
My Jun-Air by comparison is about 40 dB
I hope this helps
Mike
“Wisdom is the right use of knowledge. To know is not
to be wise. Many men know a great deal, and are all the greater fools
for it. There is no fool so great a fool as a knowing fool. But to know
how to use knowledge is to have wisdom. " Charles Spurgeon