Maybe I should suggest turning it off to the ceo. Actually, it must run all the time. Its demand cannot wait for a sound control system to turn off. At anyrate, quiet would be if outside in an open field if a person stands more than 50 feet from it they should not be able to hear it.
You will want to check out Leuven University. http://www.mech.kuleuven.ac.be/pma/research/mod/topics/topic03/default_en.phtml
Papers from Sas R. Boonen are quite good