Damper noise is rarely mentioned at the quotation stage and almost always raised afterwards. It comes from three places, and they have different fixes.
1. Blade vortex shedding
Gas passing a blade trailing edge sheds vortices at a frequency set by velocity and blade thickness. When that frequency matches a structural natural frequency, the blade resonates and the noise is loud and tonal. The fix is to change the trailing edge: a blunt or chamfered edge shifts the shedding frequency, and a splitter on the edge breaks the vortex up. This is a design change, not a site fix, so raise it at the enquiry if the duct is close to an occupied area.
2. Throttling noise
A damper at a small opening turns pressure into velocity at the gap and the result is a high-frequency hiss or a low-frequency roar depending on the pressure ratio. Throttling noise is worst at 20 % to 40 % open, which is exactly where a control loop spends much of its time. If the noise matters, the answer is usually to run two dampers in parallel so each operates in a quieter range, or to accept a larger damper that throttles less.
3. Structural radiation
The duct panel radiates the noise the gas carries. A thin unstiffened panel at 20 m² is a loudspeaker. Stiffeners, a heavier gauge or external lagging with a mass layer all help, and this is a duct design issue rather than a damper one, but it is the cheapest place to fix a noise complaint once the plant is running.
What does not work
Fitting a silencer downstream of a damper that is generating the noise. The silencer is sized for the duct and adds pressure drop that the fan may not have. It also collects dust, which on a flue duct means it becomes a maintenance item that nobody maintains.
What to tell us
Gas velocity, duct section, the normal operating position of the damper, the pressure difference, and the noise limit at the nearest occupied point. With those we can say whether the damper is likely to be the source, and if it is, whether the fix is the edge profile or the operating position.