If you have velocity noise at the return filter grille, it could be a
restrictive grille or a restrictive filter.

If you have a high efficiency filter like a washable plastic
electrostatic filter or a pleated filter, they have a high resistance
to air flow. Some of the air will bypass the filter if it is not a
good fit in the grille or holder, causing a rushing noise.

If the fins on the grille are 1/3 inch apart, common on builder's
grade, they tend to be noisy, especially if the area of the grille is
less than 200 Square inches per ton. A 20 x25 grille will usually get
noisy above about 2-1/2 tons. You can put in a steel grille with 1/2
inch fin spacing or an aluminum grille with 2/3 inch fin spacing to
reduce air noise. Or you can install a larger grille or an aditional
grille.
If the vanes on the grille are opposed to the direction of air flow in
a duct, you can turn the grille around to reduce noise

If you add baffles, you will reduce air flow, which will reduce the
capacity and efficiency of the system. If you think you have too much
air flow, find a contractor with an air flow hood or hot wire
annemometer who can accurately measure air flow. Do not reduce air
flow unless you are SURE that you have too much.

I am emailing you an article on high efficiency filters that I wrote.

Stretch