3 zones, damper whistling

[reen]

Hello-

I have a fairly new home, 7 years. Zone heating and ac Carrier unit with attached humidifier. A couple months ago i started getting a whistling sound when only one zone is on. If all 3 zones are going, whistle goes away. Called local hvac contractoor in June, he said it is pressure build up and my humidifier should be taken off, re-ducted in. What is odd to me is humidifier is not cause of the noise, i pinpointed noise to one of the dampers, but can't figure out how to stop noise. I attached pictures, the duct on left front is the one that is the culprit. The contractor told me he is super busy in June, I have called twince, keep getting pushed aside, so I am wondering if he really doesn't now what is wrong. I know this doesn't seem like end of world, but the whistle is annoying and i just want it to work properly. Just looking for direction, does this sound right? Should i call someone else? Am I being steered wrong?
Thank much.

[jtrammel]

You should have a bypass damper that opens when only one zone is open to relieve the pressure in the supply plenum

[reen]

The damper whistling is the bypass damper if I move the arm around it changes whistling noise