I want to get into the industry and have been helping friends out, but this is the first time I'm dealing with the furnace rather than the AC unit. Can someone help me diagnose this problem? The furnace suddenly become very loud (makes a low rumbling sound) when the fan is turned on at the thermostat. You can view a cell phone video my friend posted of the fan here: . What do you guys think? I am probably going to recommend him getting a professional to do the job, but don't want him to get ripped off.
Note: In the vid, He is pressing the door switch on, then off, then on again - it is not doing this by itself.