It's probably a wound heating coil, the coils push against each other at the AC frequency. It's just like transformer hum from large power transformers.
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.
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.