Quote Originally Posted by jack14r View Post
some tank(water,black,gray) level sensors have air pumps that run every so often,I think that they sound like your description,they usually only run 10-15 seconds every 10 minutes or so.
This is continuous. It cycles over and over. It is not so loud as to be annoying when the TV is on or when the AC units are blowing but in the still of the night or day for that matter, it never stops. It is a mystery!