3-1/2 ton Lennox HPXA15 heat pump making loud humming noise. Unit is cooling but has started making a louder than normal humming noise on the outside unit. Fan seems to be running slower than normal.

I have to ask, how large ids your house? I have a 2 ton Lennox for my 1600 sq foot house. I was just wondering if perhaps it was sized too large and it is "tired" of short cycling.

Check the dual capacitor. If it has failed the cell serving the fan motor, it could be causing the motor to run slower than normal and be noisy. The motor itself could also be failing.