why when i sit at an idol and turn my heater blower/fan on, my truck begins to run rougher(almost like a scrapping sensation). its reallyvery suttle and spouraticbuts its there. when i turn the fan off it goes away. i turn it back on and it returns. now there really isnt a way to tell if it happens while im driving because of its suttlety. it also happens in park.is this normal for an aged truck, is it sign of a fan motor going, or could it be somthing else. either way it doesnt seem right to me and im once again troubleshooting other issues. thanx merry x-mas

 

It's possible that your heater fan is out of balance.
Is it worse on higher fan speeds?

 

no chevylover, its the same no matter what speed. to me it seems like its drawing a lot of power. could i maybe have a bad/weak alternator, i do hear a sort of a whine, but that seems to come from the inside of the engine, it is a 92 and i dont have any other electric problems. if the fan is unbalanced can i fix/ straighten easilyor do i have to replace the unit.

 

If the problem is the fan, you would probably have to replace it.
It's unusual for something to go wrong with the fan unless something gets sucked in to damage the fan blades.
I also wonder if the problem might be the fan motor, which is a more common failure.
You could remove the fan motor and fan assemly then run a jumper from ground to the fan motor mounting area. Turn on the fan while holding the motor so you can feel for a vibration.....just a thought.