Re: a popping noise
Could it be the Q-trac chain slipping? Check it by stomping the brakes, adding gas in gear. Loud pops call for immediate replacement of the chain. Breakage while the truck is rolling will grenade the case! Use only a Borg Warner replacement part, the cheap chinese ones only last a few thousand miles.
Anaheim Gear, in Anahiem CA. has the good ones for around $150. The cheap ones are around $60. A good local trans parts house should be able to get one too.
Hope this helps, good luck!
P.S.,, They are easy to change in the truck by splitting the case. Get a good manual if you haven't done one.