Does it sound like a mule braying??? We use to have one of those Freightliners, and the same exact thing happened in it... It drove me nutz!
If I remember hearing right, the air ducts were stopped up. (with what, I don't know.) The shop told me not to worry about it. (In other words...they weren't going to fix the problem.
) In our truck, there was no air coming from the heater. Just defroster. They finally got rid of that sorry piece of
junk.