While on my last trip pulling my RV (if on the road for several hours) I noticed that when I exit the highway and slow down, I many times hear a loud roaring sound. Then when I take off again (for example from a light) again this roar. When I get back up to 40 or 50 mph and above it quiets down again. I don't think its the engine itself, the sound is like several blowers coming on at once. Is there an extra fan that kicks in when the engine is overheated? Could this be the sound?
Thanx, folks

Fan clutch. Welcome to the club. It also kills your MPG. All the info you could want on the problem.
Won't let me put a link in:
goto cool my duramax dot com all one word. You don't have a heating problem like earlier Dmax, but still have a fan.

"de fan! de fan!" ... sorry, couldn't resist the Herve` Villachez quote. :rolleyes:

Your fan is making the roar. It goes away. What the mystery is, at least to me, is why it's so intermittent. Mine does it on cold mornings for a bit, then goes away. It also does it during warmer weather at initial start up when the engine is cold. Nothing to worry about.