First time posting. Have used many of your advice before, so thank you!
My 1999 D2 with 130,000 miles, had a knocking sound. Checked the oil level and everthing seemed fine. Drove it about 10 miles and the knocking became more of a crackling sound. I parked in a driveway (up hill) and the noise became much louder. Turn the engine off and notice there was a big oil leak coming from the front of the engine (where the sound was coming from). Any advice of what it would be?

 

Loose harmonic balancer, oil pump, front cover, rod bearing, etc, etc. Could be many things but responses will be more helpful if you get under the truck and poke around a bit. If the "big oil leak" is a new thing, finding the source is a good start. Take pictures and report back.