Hi Yesterday, out of the blue, the truck while driving at speed on the interstate, started roaring. It seemed that suddenly the engine had a great load on it and was attempting to climb a large hill, none of which was the case. I noticed that rpms were about where they usually were as were temps. After thinking about it, the last time I saw this was in the 80's..and it was a fan clutch on my GM diesel. My fan clutch has so much resistance stone cold that it can barely be turned by hand... I would consider it "a ton of resistance". Is my diagnosis correct on this fan clutch? If so, is NAPA a good source of clutches?