Wen I turn on the heat or defroster in my 2001 Audi A4 after a few minutes it begins to make a loud noise that stops as soon as I turn off the heat. Could it be something simple like needing lubricant or something dreadful like a bearing for the fan…
“Loud noise” is not very descriptive.
Can you tell us what type of loud noise you are hearing?
Is it a whine, a knock, a click, a screech, a grinding noise, a…?
The new fan is not the expensive part. Getting to it us usually time consuming. Don’t try to sneak by thinking all you need is lubricating the fan motor. However it is possible something is now stuck in that fan area.
Depending upon the type of noise, it is possible that the problem is the Resistor Pack, rather than the fan motor. Now, if the OP will just clarify the “loud noise”, we might have something more to go on.