What could be the cause 'of a screeching sound when heater is on?

When the heater is 'on' or blower? If it is the blower motor the bushes on the spindle are worn out. If easily accessible spray with lube and or lube grease.

Possible A/C compressor/drive belt probem. Compressor does engage in certain heat mode selections. Should do this when A/C is on also. See if it still does this when floor output position is selected. Seek a repair shop.

poss bad fan motor