The return sounds more suspect, does the 20x8 come from both sides of the drop or just one (IOW do you only have 160sq" of return)?
International mechanical code calls for 6sq" of return (210) for your heat pump.
If you have an ECM motor, taking the blower door off is not a valid test.
Joe

ECM blowers adjust to try to achieve the desired CFM and static. Pulling harder due to inadequate return also makes them push harder.
That said an installer that is ignorant of return requirements likely shorts supply side as well.
j

The strange things is that 3 contractors, 2 of which are highly rated in my area, all said the duct work was adequate and wouldn't need to be changed.  I forgot about checking the return out last night but I'll get a chance tonight to scope it out and hopefully get the pictures to upload too.  Thank you for your help so far.

Sorry it took so long to post again, but I did a couple more tests and I think I've eliminated the duct-work as the problem. As a quick recap - Stage 1 doesn't whistle, but Stage 2 does.

I put the blower jumper on "-" which reduced the CFM to 1020 in stage 2. The whistling was still just as present as ever, which confused me because I would have thought it would have been lessened. Then I put the blower jumper on "+" which increased the CFM in stage 1 to 1000. Stage 1 still did not whistle even though the CFM was essentially the same as with stage 2 on "-". Then when it kicked into stage 2 my wife initially thought the whistling went away, but then we discovered it was just being masked more by the louder sound of the air through the ducts.

Am I safe to assume then that the duct work probably isn't the problem? Any ideas of what might be the problem? My first thought would be that the whistling is coming from the increased speed of the compressor somehow. I've thought of recording the whistling noise as suggested before, but I've had issues getting files to upload here.

It seems we need to establish where the whistling is located/the loudest, and what kind of whistle it is. My ground loop makes a high pitched "rushing" noise at the manifold but it's pretty quiet. Is yours definitely an "air" whistling noise? Maybe you can post the sound to a facebook account or even youtube (free) so we can hear it?

If you're near the cabinet, can you tell if it's coming from the blower part or the compressor part? Does it sound louder in any of the vents compared to other vents? I wonder if your system increases the loop flow in Stage 2 and whether that might increase noise somewhere.

Your problem sounds familiar.
It might be classic discharge gas pulsations that have struck some sort of resonance frequency, thus amplifies itself. Try pushing down or against the copper discharge line coming out of the top of the compressor, may be 3-5 inches away from the compressor (might be hot) and see if the pitch goes away while pushing.

had a bad txv once sounded like a tv test tone frm the old days. dogs climbimg the walls......

Just when I thought I had eliminated the duct work, I made a new discovery. I took off both sides of the blower walls to try and find the area the whistling was coming from and the whistling went away. Put the doors back on and it was back. Took them off again and it was gone. Strangely I had tried it without the one blower door prior to this and the whistling didn't go away. It never occurred to me that I might have to take the wall off the other side too. I then put the doors back on and disconnected the return duct and the whistling was gone again, so just as Joe suggested the return duct work appears to be the issue.

By my calculations I have 198 sq" of return (20x8 one direction and a 7" tube the other). I could fairly easily add another 7" or 8" tube alongside the current one giving me about 236-248 sq" of return, but I have a couple questions.

I'll call the room I'd like to add the return vent to the Boy's room. The Boy's room is a large 24'x16' room that currently has a large rectangular supply duct that supplies several vents (kitchen, laundry room, etc), including 2 vents in the Boy's room. The Boy's room currently has one 7" return vent coming from it. Would I be adding too much return to the room by adding a second 7" or 8" return vent? I don't want to suck in any outside air by adding too much return.

There is also a hall-way that doesn't have any registers, but gets heat from the upstairs by an open stairway, and from the bedrooms when the doors are open. This would be another fairly simple place to add a return duct, either in the wall or ceiling. Should I consider this instead or in addition to the Boy's room?

Thank you again for all the help. I've learned a TON both in general and about my specific system.

Is there a thermastat in that hallway?

No, there is not a thermostat in that hallway.  The hallway I'm considering adding the return vent to is in the basement.  The basement is finished w/ 3 bedrooms and a bathroom, and there is an open stairway up to the main floor.  The thermostat is on the main level and not near the stairway.

Are you thinking the hallway is a better option than the Boy's room?