the damper (活门), is only 4-6 inches away from the floor register (air out-let) .each air out-let have one damper.it is uesd for room's air balance.
to check, remove the floor register, weare a pair of glove ( the damper is a peice of thin and shape matel),put your hand into the air duct, till feel the damper (活门), then check it if loose,if it is loose. when the blower fan is running,the damper flaping back and forth, the noise is form it.
You can move it to open position more. make sure it is not blocking too much of the air flow
to check, remove the floor register, weare a pair of glove ( the damper is a peice of thin and shape matel),put your hand into the air duct, till feel the damper (活门), then check it if loose,if it is loose. when the blower fan is running,the damper flaping back and forth, the noise is form it.
You can move it to open position more. make sure it is not blocking too much of the air flow