I'm a homeowner with a heat pump problem. My service tech has been here twice, but he's not sure what it is. He checked the charge (it's fine). I'd really appreciate your input to help us diagnose the problem, since he doesn't want to just start replacing parts.

Background: All-Carrier system (1997 vintage) with 2.5-ton heat pump, air handler and electric heat strips. The heat pump ran fine all winter. On Saturday I switched it to cooling for the first time this year. I noticed the following problem:

- Heat pump starts normally and cools fine for the first 20 minutes or so (my tech checked the output temperature and said it was fine).

- After running for about 20 minutes, a hissing noise can be heard from inside the heat pump (like refrigerant flowing in the lines, only louder). This noise gets progressively louder over a 10-minute period, until eventually it is quite loud and can be heard from 10 feet away above the fan and compressor noise. Along with this hissing noise, I noticed that the air blowing from the top of the heat pump, which is normally warm, gets progressively cooler until it is the same as the ambient temperature.

- Eventually (after a total running time of about 30 minutes), the compressor and hissing noise both stop. The fan continues to run (since the thermostat is still calling for cooling).

- At some point after that (say, a few hours - I'm not sure), the compressor restarts and the same cycle repeats again.

That's it... To me, the hissing noise suggests some sort of gradual pressure build-up inside the heat pump, eventually causing the compressor safety mechanism to kick in. Trouble is, we're not sure what is causing the build-up. My tech said it might be the piston in the reversing valve gradually shifting due to a leak or crack in it, but he's not sure.

Any ideas? We'd really appreciate your help on diagnosing this one.