Heater fan runs on high, some times, but doesn.

Heater fan runs on high, some times, but doesn't run at all in any other position. Replaced the resistor and fan ran in all positions for a brief while, but with a loud, intermittent sound like the damper door were fluttering. When the fan quit running this time, it seemed like an overheated or electrical smell came from the heater control. Any idea what's causing the resistor to go, the intermittent sound when the fan runs or checks to perform on the heater control?