We have a new home with a split 13 SEER HVAC heatpump system. The lineset is probably 50 feet long. In the cooling mode several minutes after the airhandler stops running, I sometimes hear a knocking sound which seems to come from the lineset which runs down inside an interior wall from the attic into the crawlspace and outside to the compressor. The knocking is about 2 beats per sec and only lasts for maybe a minute. It only seems to happen when the outside air temp is cooler than 70 degrees and is not at all repeatable. 4 AM in the morning is a typical time to be awakened and hear what sounds like a knuckle tapping a drywall.

That is why I never run a lineset in a wall cavity. This noise is caused by the refrigerant line set. It is refrigerant expansion after the unit it turned off. The only way to fix it is to run the lineset down the outside wall.

You can add a liquid line solenoid valve to the system that will solve the problem. It keeps the refrigerant lines from equalizing on shut down and you from hearing it(its a 100% shut off when the T-stat quits calling for cooling). They should have installed one from the start with that long of a line set and going up a inside wall. As time goes on it will only get louder and more frequent.

You can add a liquid line solenoid valve to the system that will solve the problem. It keeps the refrigerant lines from equalizing on shut down and you from hearing it(its a 100% shut off when the T-stat quits calling for cooling). they should have installed one from the start with that long of a line set and going up a inside wall. As time goes on it will only get louder and more frequent.

Thanks very much to you both for the tip; I will pass this on to the warranty folks.:D