Heat Pumps and Electric Heating - Rheem Outside Air Handler squeaks when very cold outside

I have an older (circa 1991) Rheem heat pump system installed at my home. Operation and performance is normal for the winter, except for the following: when the temperature drops below around 32 F, I hear a chirping noise coming from the outside air unit. As the temperature drops further, it becomes worse - more akin to a sqeual. It's not terribly loud, and, as I said earlier, is confined to the colder nighttime temperatures. The unit does not have a belt.

Any thoughts?

we all tend to make noises that wern't there when we were younger...especially our cars. You could try take a 5/16ths nut driver with you and attempt to tighten the screws a bit. Likely the cabinet sheet metal is shrinking during colder times and rubbing on another piece of sheet metal. The only concern is the lubrication of the condenser fan motor. Some actually have oiler ports, and if yours is one, you'll need to oil up the motor. Now days almost all are sealed ball bearing (lifetime lubricated). The OEM Lifetime was calculated as when it stops working. :o

those would be 1/4 inch screws on a Rheem, those old units may even need a philips bit