I have a Lennox 10ACC-042 split-system central air conditioner. I had a checkup/tuneup performed on it last month and everything was great. Also it is cooling fine, no problems there.

The problem is, after it has finished running and is not active then after some period (ranging from a few minutes to an hour or two later) the outside compressor unit will emit a loud high pitched sound, possibly indicating some kind of error condition (?). If immediately set the thermostat to a lower temperature so the system will kick on, the high pitched sound will stop and the compressor and fan outside will turn on, and work properly.

Also if the system runs for a while and cools the house then if I turn both inside thermostats to the OFF position (instead of cool) then it will still make this high pitched sound outside a certain amount of time later, usually about an hour later. The sound will continue for a long time until either the system becomes active again (cooling), or if I turned the circuit breaker to the outside unit off the sound will stop after about 5 minutes. Any ideas?