Whistling noise when only one zone calls for heat/cool in a 2 zone system

[n00brh]

The setup is an EWC NCM300 zone controller on a Rheem furnace with 2 zones (12 years old). Zone1 downstairs and Zone2 upstairs with MA-ND5 dampers. There is a loud whistle heard through a downstairs vent when only Zone2 is running and similarly a whistle heard through upstairs vent when only Zone1 is running. The whistle can be heard at the furnace (in the basement) but it is faint and drowned by the other furnace noise. No whistle when both zones call for heating or cooling simultaneously.

As a test, I did the following. When A/C is running, I can get the whistle noise down by just pulling out the air filter (I use MERV 8) but when heat is running, there is no reduction in the noise even if I run the furnace without an air filter.

Any suggestions to fix/reduce the noise which is really aggravating?

[n00brh]

I read up on the available literature onsite and this forum and I setup the zone1 damper to allow for some air leakage when zone2 is running - this has completely killed the loud whistle. I am trying to understand how much I have opened the damper by doing so. Based on what my image shows, if I move the end stop (on the close side marked with I) all the way to the right, is that equivalent to 20% leakage? I want to make sure I am not opening way too much.

Also, there is a bypass duct between the supply and return that feeds into the humidifier (on the return side). There is no damper on that.

Screenshot 2025-10-26 103745.jpg

[Steven W.]

There is a gap between the damper and the duct work. Pull out the damper and put some gasket on the top and bottom of the damper and place the damper back into the duct. Whistle noise should be gone.