Hum/Resonance After Repair - How to fix?

[MTDIYer]

Our AC failed in the summer. A local contractor diagnosed the problem as with the blower motor. He replaced it, and the system works. But, he and we noticed a hum, a low resonance sound, coming from the vents. He said he would fix it. But after repeated calls, which he fails to return, I think he is gone.

What could I do, if anything, to lessen the resonance or hum? Is my only option getting another blower motor and installing it?

 

[bud16415]

Sometimes the natural frequency of one thing matches another in the system and you get these problems where you never had them before.


When it is doing it I would walk around and push on the duct work and other things that could be vibrating. Chances are you will find some point where the hum stops. Then add another clamp or a strap, block, etc to break the frequency.


Sometimes they are easy to get out and sometimes not so easy.


That is my advice as a starting point others will be along with other ideas.

 

[pjones]

Depending on the type of sound, It’s probably a loose panel or something simple that occurred while the new motor was installed, or high air velocity. Might just need to get removed and reinstalled making sure all the panels are tight and no screws missing, or airflow adjusted providing the airflow stays within the factory recommended specs for that unit.

On occasion I find some type of vents can cause vibration with high air flow. If the motor capacitor was failing for a long time causing reduced airflow then the sudden return of full air flow may be enough to make that noise noticeably louder. Those can be tricky noises to resolve. Typically need to remove the vents and refasten each louvre so they don’t vibrate. I’ve siliconed some before to help stop the noise as well but only on ones that are not adjustable.

If the noise can also be heard inside the air handler then I would start there since the sound is probably then just channeling through the ducts and out the grills.