While on the way back from the deer lease last week my '99 f250 sd 4x4 5.4l started a grind sound that would come and go randomly. Lasts about 1 second. Seems not to change the way it drives. The dealer had it for 2 days and couldn't find anything. This happens at freeway speeds 55-70 mph. Any help would be great. thanks Huckster

 

Sounds like your AC compressor kicking in, it is designed to cycle while in Defrost. My other thought is a hub lockout, but you would really feel that in the steering.

 

I thought about both of those also. The dealer checked both out and all is ok. My wife heard it and thought it was in the rear section of the truck. That makes me think I may have a small pebble in the rear brakes. I just don't want to be stranded in the middle of nowhere in the middle of the night with a truck full of grandkids.