My HVAC system sounds like a helicopter that is flying overhead. I know that probably sounds wacky, but I don't know how else to describe it. What do you think could be wrong with it? We just had it installed a year ago.
I'm not an HVAC specialist or anything, but anytime I've run across that sound it indicated that something was wrong with the belt. I'd go ahead and call someone to get it looked at. You don't want your heat source to fail this time of year.