W900L ghost howl

I have a 99' W900L 72" aerocab studio, 550 Cat, 18spd that I've owned for about 10 years. Every year around this time it develops a howl that seems to come from the drivers side under the dash or firewall area. It comes and goes daily but I cannot duplicate it at will, seems to be only when it's cold outside. Just something that has always eluded me.

 

No, thanks for the help though. It's done this as long as I've owned the truck. Definitely not exhaust leak or any of the other comments I received. I just assumed it must be a KW thing, it doesn't speed up or down, get louder, faster or anything. I used to think it was the heater years ago because it only seems to start as it gets colder outside but it does it with it off also. I have completely rebuilt this truck front to rear from engine, 7" complete exhaust system, air to air cooler, radiator, a/c system, transmission, clutch, flywheel, entire driveline including both rear ends, wheel bearings, etc and everything in between. Didn't know if anyone else had this ghost sound???