Hi! Yesterday I went to my parents house and noticed that the refrigerant line from heat pump to furnace was covered with about 1/4" of frost, from furnace to the inside of the heat pump. It also seemed to me that the compressor's noise was a bit higher pitched than usual. I told them to shut down the system and wait until today to restart their system, and to monitor the presence of frost on the lines.

The heat pump is 23 years old and has worked great so far, and it seemed to be working fine when I looked at it today (it had been running for about 3 hours when I looked at it).

Could you please tell me what is the likely cause, if it's likely to have damaged the system, and if it means the system should be checked. About a week ago, they were complaining that the basement was freezing, so I closed off about half of the basement registers. Can it have something to do with it? Thanks.