I get this popping noise when I'm around 25 mph sounds like its when it shifts. But it might only do it in 4x4 ill have to double check. Any ideas? its a 91 k5

 

Hmmm....Popping from where? Engine compartment? Drive components? Exhaust? Where does it sound like it's coming from?

 

Well it seems to be coming from the tranny/tcase area. only hear it really in 4wd. but it does kind of pop or make a noise when shifting into gears sometimes. its a 91 fullsize auto 350 w/ 155k miles. Im afraid to flush the tranny in case junks holding it functional

 

Sounds like it might be universal joints. After a drive try touching them and see if they're hot. You cannot always see the play in them when they're bad.