coil cleaning, replacement.. musty smell.

I have been having trouble with a musty smell coming from our ac vents.. Just when the air is blowing from them.
I have cleaned the coils (DIY.. I know you cannot recommend this but I've done this previously), and sprayed window/door spray foam to seal where the lines go under the slab to the outdoor heat pump. ("..…").
Nothing is wet in the air return which is directly under the air handler inside our home.
We had a company come clean the cents, and coil, about 2 years or so ago. You'd never think this coil had been cleaned ever, even after my DIY job.
When I open the air return and remove the filter so I can see the inside, it smells musty in there. I'm sure that's where the smell is coming from. I've cleaned that out too.
The unit is a 2005 I believe, and is Carrier. It is Puron/410A.
We have tenants moving in a week. So I'm pressed for time now.
Do you think the coil has to be replaced, or cleaned (again?). In looking up cost to replace coils, it almost sounds like it makes sense to replace the whole air handler because it seems astronomical. (Not asking for pricing, I know that's a no-no).
TIA