Ten year old Bryant heat pump cools house fine, but the unit doesn't run as quiet as it once did. Inside the house you can hear a slight humming. Seems to be getting a little louder as the days pass. When you go outside to the unit it sounds normal, but when the fan stops and the 20 seconds or so it takes for the unit to completely go quiet there is a buzzing noise like something's decompressing. Any ideas?

Could be the reversing valve is getting noisy. The valve is very expensive to replace. I would just let it run if you can stand the noise.

You say reversing valve. My unit is gas and I think reversing valves are only on electric units, correct?

Wow I am glad you gave that information in your first post. Gas units are very rare in these parts. I used to service Servel units but not Bryant. Also serviced Carrier units which is about the same as Bryant.

If you have a gas heat pump it circulates ammonia in the sealed system. Then it chills the water or antifeeze mixture.

Sorry but working on that equipment is for professionals only. I recommend you call for ON SITE service if you believe your having a problem.