I have a Coleman Evcon electric furnace in a mobile home it run fine on AC and the ON position but when you put it on auto it will set and humm I have to spin the fan by hand to get it started. At first it was only a few times but now it is every time it startes.

Try replacing the capacitor bolted to the side of the blower. These can be ordered through partsdirect, they are titled by voltage capactity. I recently had to do this to fix my blower humming but not coming on.