I have a Lennox thermopump which, when turned on to heat: the fan turns on, the compressor does not turn on and there is no hot air generated. When turned on to A/C: again the fan turns on, the compressor does not turn on and there is no cold air generated. My husband and I opened the heat pump casing this weekend and found the following: a hot scroll compressor and a buzzing sound coming from the compressor. We turned on the heat pump to A/C, the fan turned on but a buzzing sound came from the scroll compressor. (I have a video clip of the buzzing sound if anyone is interested)

The compressor is the heart of the system. It pumps the refrigerant providing both heating and cooling (depending on which way the reversing valve tells it to go). Your compressor sounds like it is stuck. This could be as easy as a malfunctioning start control, or as bad as a broken compressor. The only part that confuses me is the fact that the breaker or fuse has not tripped. Normally, a stuck compressor will trip the breaker in the panel. Please double check to be sure the noise is coming from the compressor itself and not a buzzing contactor (the electronic switch that turns it on). Best thing I can say is to check the breakers. If they are tripped, re-set them and see if the unit comes to life. If the breaker trips again, or if it is not tripped at all, call your local Lennox dealer, because a broken compressor is not something the average homeowner should fool with.

Which fan the inside fan or the outside fan?

To Doc Harrison's observations: the breaker/fuse did not trip. The noise is coming from the compressor as far as our ears can tell. We heard the buzzing sound once we pushed the black switch inside the thermopump located above the compressor (I guess it is like a test switch for technicians to see if the thermopump goes on).
To hvac1000 question: the outside fan turns on once we pushed the black switch located insude the thermopump above the compressor.
I have a video clip of the "buzzing sound" if anyone is interested

Pust the clip

I am trying to post the clip but my .avi file is too big. SO I have uploaded it to fileup123.com. Just click on the following link and you will access the thermopump file (MVI_0764.avi)
http://www.fileup123.com/uzinokcafl.html or if you prefer the zip version acces the following link: http://www.fileup123.com/txyabwtsfl.html
Let me know if you have any problems. Thxs

Sounds like a relay. When you try to start the unit put your hand on the compressor to see if it is making the noise. If not have your assistant activate the unit until you find the item making the noise. You might also check all the wires going to the compressor and make sure you have voltage on both sides of the compressor starter/contactor when activated.

It does not sound like contactor chatter from a low voltage condition but you never know.

A couple of quick questions:
1) is the compressor starter the same as a capacitor? Is that where the fan and the compressor are connected to a capacitor?
2) If I do put my hand on the compressor, will the noise decrease as a result or will we feel the compressor vibrate if it is the source of the noise? How will I know the noise is coming from the compressor by putting my hand there?