Driving down the interstate between 55 and 75 mph the engine will begin to howl. I'm not nailing the gas, just driving along. This happens quickly, but not abruptly. It's as if I hit the gas to pass someone and the tranny is shifting down into a passing gear. If I had OD, I would swear that the tranny had downshifted or was in a passing gear. Of course I do not have OD. I have slowed down (it sticks in the "passing gear") and shifted into 2nd, which works, so I don't think that the tranny is shifting down. There is a noticeable drag (almost like being in 2nd) when this happens. Nothing else seems wrong, although this did start after my last run on the rocks. I have driven the truck about 5,000 miles since then, but never noticed it before so it may or may not be because of something done on that run. '90 Burb. 350/TH400. 100,000 miles. All original drive train. No smoke/burning smell. WTH??????