Depends a lot I guess on what brand, model of unit you have there as well as the drive system for the room air fan motor. I suspect this one has a belt drive and that the belt is stretched some and slipping under loadup which would make the very noise you refer to. If the fan does not run at thr right speed it can make the heater cycle on and off at short intervals by operating the overheat device instead of the thermnostat. Best you get a serviceman to check this unit over and make sure it is set up correctly.
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