I have a 1998 ram 1500, 318, I have been noticing a howling, growling type noise coming from under my truck. This noise happens in all gears except neutral and reverse. The noise does not change with different speeds either, but the truck has to be moving in order to make it happen. How I know it doesn't do it in neutral is because I put the truck in neutral at to top of a long hill and coasted all the way to the bottom and it was quiet, I then did the same thing but left the truck in drive, both with the O/d on and off and it makes noise the whole way. I have checked the rear differential fluid level, it is good, and have cheeks the transmission fluid level, it is also good. I am lost as the what my next troubleshooting step it to narrow down the problem. Any help is appreciated and I'll answer questions to the best of my knowledge.