Air handler squeals
Squealing sounds from a forced-air heat pump generally occur when the belt that connects the motor to the fan slips. If the blower is making a grinding noise, shut off the unit and call a heat pump repair technician --the motor's bearings are probably shot.
In most cases, the belt is improperly aligned or worn and needs replacement. Follow the instructions in your owner's manual (if you don't have one, contact the manufacturer and ask for one).
This job involves removing the access panel, loosening a couple of bolts that hold the blower motor at the proper tension, and realigning or adjusting the belt. In many cases, it's a good idea to remove it and buy a replacement at the hardware store.
When reinstalling the belt, be careful not to over-tighten it--this can wear out the motor bearings.