Is the noise from the actual blower, or from the airflow created by the blower?

 

Does not seem to be the blower but seems to be noise from the air flow.

 

The best solution would be to adjust dampers where the branch duct (flex) connects to the trunk.

However, dampers are often not installed simply because it takes extra time during the installation and installing contractors do as little work as possible. If that is the situation, the solution would be to have dampers installed so the air pressure on those ducts could be reduced.

 

Oversized equipment on undersized ducts.

 

Oh c’mon. That never happens. 🤣

 

In my experience as a system and duct designer, and fabricator: Duct air noise that seemingly impossible to locate, is most likely mechanical noise of the blower being amplified by the sheet metal ducting, this is especially the case with branch lines close to the blower and especially so when the branch is stove pipe with no flex. The solution to all duct noise is applying duct liner (inside fiberglass insulation) to the supply plenum and the return boot at the blower location. If done correctly there is no need of insulation pins to hold the insulation in place. Done correctly requires applying a non water based spray or brushed on glue liberally to both the sheet metal and the insulation. At the very least if one installs insulation on the inside top of the plenum and inside back of the return boot, 95% of the blower noise will be mitigated. Best practice is lining the inside of the supply and return plenums, something rarely done with residential installations.

 

But, in my experience, is always done in commercial installations.

. . .
At the very least if one installs insulation on the inside top of the plenum and inside back of the return boot, 95% of the blower noise will be mitigated. Best practice is lining the inside of the supply and return plenums, something rarely done with residential installations.

 

Thanks to all who have replied.

Question for AtticAce: The problem areas are just two vents on the second floor which are close to directly below the blower which is on the third floor. The ducting is flexible not metal. Without going to the major expense of breaking open the second floor ceiling to access all full ducting am I likely to get benefit by (1) having the flexible ducting leading to these two second floor vents replaced with an insulated flexible ducting as far as this is accessible before it goes below the third floor to the second floor, and (2) installing duct liner to the metal boxes behind the two vents.

Thanks,