i really dono but what do u mean by Fan type hum, or electrical.Would the hum be in the 60 Hz range?... Any chance you could send a sound sample? Fan type hum, or electrical?
Aren't the coils moving?If there are no moving parts in the heater (like no fan), the quiet humming that you hear is because the coils inside are expanding and contracting at 60Hz, as the 60Hz AC current flows through it. THat's why light bulbs, transformers, and power boxes seem to hum even though they have no moving parts.