York H 1 RA 018 S 06 - Loud knocking in wall when AC turns on

[GoldenBear]

Over the past few weeks, there has been a very loud knocking in the wall when the AC turns on. The unit is outside. The knocking is near the ceiling on the second floor. It's less then 12 inches from the ceiling. It's a new sound and rather loud. The knocking goes away after maybe 30 seconds. It may happen during the day, have not noticed it. But I certainly notice it at night (it's in my bedroom). I can't make the connection between the AC and the knocking.

Note, the unit is approx 11 years old.

Thanks in advance for you help.

[ACtechGUY]

This is what is most likely happening.... The Liquid refrigerant line is expanding when the line gets warm.

Over the years that liquid line (3/8 diameter copper line) has expanded and contracted and flexed many times. That line is now touching a piece of wood in your home. As the pipe heats up it will expand(it is copper and will always expand when heated). As it expands it jerks forward making a sound that may sound like knocking. Once the pipe is done expanding (after 30 seconds or so) it will cease to make a noise. If you paid very close attention you may hear pops/ knocks or creaks after the system shuts down, due to the pipe returning to it's original size.

I can offer no other possible reason for your noise.........besides the noise and temperature change is upsetting the rat that is living in your wall right next to your bedroom.

[GoldenBear]

thanks TechGuy. I think you nailed it. Very much appreciated.