I don't think your plan is going to block a noticeable amount of
noise

I once saw an episode of TOH where the owner had to deal with street
noise - i.e. traffic.

The landscape designer suggested a very low stone wall along the front
of the lot. When questioned by one of the crew, she said that since
most of the noise comes from the tires, all it would take is a very
low wall to lessen the noise considerably.

Now, I'm not saying that everything they do or say on TOH is 100%
correct, but I would think that something like that would have been
verified before it went on the air. I can picture any one of the TOH
guys throwing out an off-camera "Huh? Are you sure?"

So, if a low wall will lessen traffic noise "considerably", I'd think
a tall wall around a AC unit would certainly have an impact.

It would need to be a very *solid* tall wall. A small stockade fence
will just pass the sound through. A small brick or stone wall, not so
much.

--
Steve Bell
New Life Home Improvement
Arlington, TX USA