weird knocking noise 1991.5 cummins
 

I have a 1991.5 first gen auto trans, around 150k miles. Runs great drove it for 3 hours at 70 mph most of the way and never missed a beat. About 3 days a go I had to put it in 4wd and forgot to unlock my hubs and drove about 15 miles on the highway before I remembered. I unlocked them and all was fine for about 10 miles then stopped at a red light and heard a thumping sound coming in and out (believed to be by the back of the motor). When Started driving again it went away???? Any ideas??? The only thing I did different before it happend was add a little transmission fluid. PLEASE HELP DONT WANT IT TO BECOME MORE SERIOUS!!!!!

you have a NP2-5 transfer case?
mine "hydro locks" need to spin the tires or put it in reverse, back up, no breaks and shift back into 2H

I disengaged the 4 wheel drive just never unlocked the hubs.