Had a defrost module replaced and the service man put 2 lbs of gas in the unit a week ago.. now the air handler sounds like a freight train coming through the house when the unit come on and sometime when it shuts off--it does not do it all the time and we can not duplicate it by turning it on and off at the switch or therostat--of course when the service man came back it would not make the noise and he thought we were crazy--it's so loud that it will wake you up at night--hhhhhhheeeeeeelllllllpppppppp!! Please what could be wrong??

When did the noise start - after the Freon was added? How was the system performing prior to this? 2 lbs. is a substantial amount of refrigerant to add - maybe too much in your case.

Started after the freon was added.. think it is building up too much head pressure on the compressor but would you hear this inside the house through the air handler?? Thanks...

In the heating mode, the hot gas discharged from the compressor goes directly to the indoor coil.

Scroll compressor?

Hello,
I am having a similar problem. The system heats fine. After it cycles off for a while, the air handler makes a terrific noise for 20-30 seconds. It is a loud rumbling sound. Wakes you up at night. We had a HVAC guy come out and, of course, the noise did not occur and he gave us the look of disbelief. He checked the condenser and said the wrong size motor was installed and replaced it with the correct size. Problem did not go away. The outside unit also makes a vibrating noise like this: YouTube - Carrier Heat Pump Noise Problem (http://www.youtube.com/watch?v=9gfnmVB5q3c) (not my video)
Any ideas? Thanks

Well I finally got the company to come back and let out some of the freon and the noise went away... of course they said that was not the problem but funny how the noise started when they added freon and quit when the took some out... really need a warm day to get the gas charge where it should be... now if you unit has not had any extra freon put in it you could still have a problem withit building up too much head pressure... might be worth having them dump all the freon pull a vacuum and recharge the unit... do this on a warm day... good luck

This is happening in the "heat" cycle. Would the freon level affect the heat cycle? It has not been charged since we bought the house in November. We have not used the AC since then.

Nobody with any ideas?

Does it make a louder noise after shutoff, or during defrost?

This happens at a different time than the defrost. The noise is coming from the attic where the blower is. Yes, it is louder.

Most likely, the noise from the attic is being generated by the blower - if the wheel is very dirty, vibration will result. If not corrected, motor wear will result. The heat pump in the video sounds like a scroll compressor that is slapping a bit when the pressure unloads during switch-over from heating to defrost. I have corrected many a situation like this by replacing the compressor - when the unit was still under warranty. I noticed that the heat pump coil had a band of frost in the middle. This usually indicates a low refrigerant charge, or a restriction in the outdoor distributor. I am more suspicious of a restriction, since the unit seemed to defrost pretty quickly. A system that is properly charged at initial start-up won't require additional charging unless a leak develops. Have you ever noticed frost-banding like that on your heat pump?

There is a valve in the freon line that reverses the flow direction when the thermostat is changed from heating to cooling. I was told that this valve goes bad and "flutters" when the unit starts up.