Carrier 38YXA024 (R410a, Copeland scroll), 58CVA070 (VS 80%), and CK3BA030 (R410a dedicated horizontal N-coil in the attic) setup. I know it has a field-installed Sporlan TXV but don't know exactly what kind. The charge has been checked with the superheat/subcool type method once or twice already, though in fairly mild ambients (maybe 75 degrees).
It's working fine, and has all along. I started to notice through the winter, though, that about a minute after the end of a call for heat, the outdoor unit would start to make a VERY high-pitched ringing or whining noise for a minute or two. It's pretty loud, and it does so very consistently at the end of each cycle. BTW, I know what defrosting is and when it is defrosting, and this isn't that. It's only after the compressor itself shuts off after running for awhile.
I wondered if it was just an quirk of heating mode, but now we're well into AC season in Georgia and it's doing it after cooling cycles too. It's fairly annoying, and a bit worrisome, since I don't remember it doing so last summer or fall (the system was installed in August 2004 or so). Is this something to be concerned about, or is it normal?