Heat Pumps and Electric Heating - Noisy heat pump changeover valve

I have a Tempstar 5 ton heat pump. When it's cooling and the thermostat is satisfied the compressor shuts off normally most times. Sometimes however, it sounds like someone just started up a home shop air compressor. The noise is very loud at first, then gradually quiets down, lasting about 30 seconds. I've done a little checking and find that it happens even if I shut off the disconnect while the noise is going on and it keeps up, so it's not the compressor or fan. I can find nowhere on Tempstar's website to ask a question, only a referal to a local contractor. The company that installed it has gone out of business. A tech told me it's the changover valve trying to equalize and it's a normal noise. He also said that Tempstart would not repair or replace the system for this problem. I've never heard any other system make that kind of noise and since it's getting close to the end of the 5 year warranty I want to get it taken care of.

Questions: Is this normal? If not, should I have just the valve replaced or the whole condensor?

Probably a bad install and the system was not evacuated properly leaving noncondensables like air and moisture in the system. That would be my guess.