Back in the 80's the thermostatic fan clutch failed on my first Class B, which was a 1979 Ford E250 with the 351 Windsor engine. That vehicle didn't have a tach so, at first, like others, I thought the extreme roaring noise (which didn't go away) was caused by high engine revs due to transmission slippage. But I finally figured out that the fan clutch had failed. I was on the road at the time so purchased a new clutch to put on when I got home. But since I still had over a 1000 miles to go and I couldn't stand the constant roar anymore, I ended up changing it out at our campsite. As others point out, it's normal for the fan clutch to engage for a short time when the engine is cold or kick in if the engine temps get on the higher side.