Fan noise is generated from the flowing gas impeller housing vortex. It is subject to the following aspects:
A. Basic design fans (axial fan or centrifugal fan, design principles of impeller, etc.).
B. Fan model, it is related to the pressure required and flow rate.
C. Fan operating points, such as: the fan runs in the range of characteristic curve.
D. Fan speed, fan noise has different sizes in different speeds.
E. Fan casing and impeller are designed specifically according to the principles of fluid motion. Noise sizes depend mainly on the flow rate and pressure as well as fans of the model. Noise measurement unit is dB (A). Letter A represents the normalized frequency assessment, which takes into account the relationship between noise level and audio.
AC condensor outside should only have the fan come on when the AC is on, not when the heat is on, so why would it be coming on when it is below 45F?
Usually the fan is mounted on and directly driven by the motor. Fan blades do not usually make a squealing noise when wet.
Most likely the bearings have ran dry and at the very least need lubed. That it's been allowed to run like this for a while (presumably), the bearing may have worn enough that the whole motor needs replaced. Many fans today are considered permanently lubed but that is nonsense, they just don't put a lube hole in to save money and make people buy a new motor sooner. They just need pulled out, the through-bolts removed, and disassembled like this until you have access to the bearings or the felt oil reservoir rings to add a few (several, depending on the motor size) drops of oil. Standard 20wt motor oil, or even 5W20, 5W30, 10W30, etc will do in a pinch.
The only time the blades should/would squeal is if the nut that secures it to the motor shaft is loose and so the blade assembly is wobbling around.