Loud Whistling Noise when register closed

I have two ac units in my home. One upstairs, one down stairs, both installed into two different attic spaces. I closed several registers in the upstairs to try and increase air flow to some rooms that are used more. After I closed them they made a tremendously loud whistling. We not really a whistle, more like a hurricane sound. It is very loud, it can be heard through two closed doors. It is like this down stairs also only on one register. 2 upstairs, one downstairs. What can i do to stop the noise and still keep the registers closed.

 

Some units have a multispeed fan. Go to a slower speed. Or, you could replace the fan motor with a multispeed motor.

Otherwise, install damper(s) upstream of the register(s), and shut the dampers. The throttling noise will then be across the damper, so install it somewhere were the noise will not be objectionable, possibly in the attic. Insulate around the the damper.

 

I should have dampers at the main duct if a good company installed it. If not you will need to add them

 

what is a damper? how do i go about installing one? I believe that installing a multi speed motor will be too $$$ and above my skill level also.

 

A damper is essentially a valve just like in a throttle body. They are usually installed where the duct attaches to the plenum. When partially closed they keep so much air from entering the duct. They are adjustable. Based on your comments I feel you might go for an alternative I've used to reduce airflow through a register. It's not ideal but does work. Get yourself some AC ductboard...yellow fibreglass covered with foil on one side. You only need a little so raid a dumpster at a home construction site. Cut strips and install them behind the register (to look like an old Buick grille). They need to be snug so the air won't budge them. Good luck.