Not normal. Look for a suction-side air leak or low water level in the skimmer well. It could just be some blown-in debris has jammed the weir (flapper) Pool School - Suction Side Air Leaks
The heater on or off shouldn't make any difference at all, since it's on the pressure side of things. If there was a leak, water would be squirting out somewhere, air wouldn't be getting drawn in to be ejected into the pool.