It is possible that the motor is going bad and the humming you hear is the motor shaft not turning. That would explain the icing of the outdoor unit. Are you certain there are only 2 filters to change?
You need to get into the attic and check to see if that motor is overheating. It is a fairly simple changeout and you can get a generic motor online.
I'm pretty sure there are only two filters. When in the attic looking at the air duct from the inlet side, it only goes to two places in the house. This problem has occur before, few months back. It will make that humming sound, after I leave it off for awhile, then turn it back on.. it will work fine again. This time it got worst, hardly blowing any air at all. I will try to take the motor out tonight and hopefully Grainger has a replacement. I'll give you an update by tomorrow morning.
Ok. Grainger should have the motor. Make sure you make note of the wiring. that motor is inside of a blower housing that slides out of the unit.
I have install the new blower motor with a new capacitor. Everything works great, but now it is loud. I can hear humming through the all vents in the house. I did cleaned the blower housing and the blower wheel. So it's like new now. The only thing I did was take out these dust collectors or filters (whatever it is) that is mounted on the wall inside the unit around the blower housing. I took them out because it wasn't glued to the wall very well and part of it was suck into the blower's wheel. I plan to put them back in once I find some new ones, not even sure what they are called. Could it be an airflow problem causing this humming?
Did you get the same HP and RPM on the new motor?
Yes, same 1/3 HP and 1075 RPM.
Make sure all is tight. The hub nut, the motor housing, the motor bracket. Make sure nothing is loose.