I have a Trane self-contained over and under heat pump, which is 19 years old. Last night the outside temperature was below about 60 degrees and I was "cold", so rather than put a blanket on the bed, I turned the thermostat up to 75 degrees. This morning, when the temperature had risen back up to about 70 degrees, the HP started making a "whining" noise. Being concerned that it might damage the unit, I called a Service Company out. It was still making the noise when the repairman arrived, and here is the explanation he gave me:

In an "old" unit like mine, when the outside temperature is "near" the indoor temperature, the air being pulled in is warm enough that it causes the compresser to work harder to try to relieve pressure and thus the noise. In some units, like Lennox, there is a cut -off valve with a reset button to take care of this. In newer Trane units, it resets itself, but in "old" units like mine, it reacts like mine did. He said he gets 5 or 6 calls each year in the late Fall like this.

My question: Is this a reasonable explanation? My thinking is that it would seem reasonable, since he could have put in parts which I didn't really need, had he opted for more $$. I live in Oklahoma, where extreme temperatures are rare - but I have never heard this sound in either of the two "old" heat pumps which are installed in my duplex. Appreciate any comments. Thanks.

P.S. - I KNOW............... I need to replace the heat pumps (one is 19 years old, the other is 16 years old) but they are over and under, require a specific model and the cost is near $6,000 each and I just hate to spend that money before I really am required to. (By the way also, the cost when the old units were installed was under $2,000 - is that inflation, or what?)