I had the same problem after having my 2 1/2 ton air handler replaced
with a 3 ton unit 2 months ago.

With the gill open the noise went away.

So, what I did, was remove every other fin from the grill/grate, problem
solved.

Keep in mind I don't have a "stamped" grill/grate. It is a heavy duty
one made out of alumium. I took the frame apart and removed every other
fin. That increased the openings from 3/8" to 3/4" between the fins. The
ONLY thing I don't like about it is, you can now see the filter but
after a while you don't even notice it.

As another poster recommended going with a commercial grill/grate should
solve the problem. Just make sure the space between the fins is greater
than what you have now.

Or, you could just (if you grill is stamped) cut out every other fin
smooth the edges with a file and see if that takes care of the problem.
You might want to try this first because those grills AREN'T cheap!

Also, to the poster that mentioned NOT to use a plastic electrostactic
filter, that is what I'm using and it cut down on the wind noise
significantly over pleated filters, while also keeping my unit, home and
nose clean. : )