[Aloha1]

Originally Posted by tuccillo

I have had something similar in a previous home with a Carrier Heatpump. It only occurred in heating mode with the compressor in low-speed (it had a two-speed compressor) and was best described as a "hum". The system had a fairly long lineset (about 75' IIRC) and I suspected the issue was that the lineset was effectively tuned to the frequency of the compressor. The Contractor had a similar theory and increased the lineset by a few feet (brazed a short piece to the liquid line) and it markedly reduced but did not totally eliminate the problem. The neighbors house was not part of the problem.

I suspect your theory is correct. I'm hearing the same type of low constant hum at nite in the bedroom. It only started when the heat came on this past week.