Playing catch up on your thread. I was thinking that since it seems to only happen at low speeds and while turning left.. have you thought about an axle? Just snow balling here so don't shoot me. If the axle shaft was broken and the break lined up most of the time, it would pop when under load such as in a turn ( I had a crank shaft do this). This could also be true if the carrier had a bad bearing or "C" clip. Just thinking...
You may be on to something. Let me try to recap what I had written out last night. Will try to write it verbatim, but I have slept, so some may have gotten lost in the last 10 hours.
Okay, I covered the lunch time incident. Lets jump to after work yesterday. I mounted the camera in the same spot. Armed with a full tank of gas this time, I hit the normal paths that this popping will happen. Nothing. I was more heavy on the throttle too, although I did some normal take offs as well. No popping. Took turns left, right, tight ones, normal ones, turnarounds, three-point-turn, nothing. Was going to stop and get some juice for me but decided to head home instead. But since it hadn't happened yet, and going home is a string of traffic lights, and I wanted a more fluent motion and the ability to turn left and right on the short segments of downtown, I retraced some steps. I drove past the post office and the next stop it popped.
That was about 16 minutes into the video. The entire time the trailing arm connections did not move at all, no slop in those whatsoever. Driveline, except for the minor wobble at the transmission, nothing. Frame, solid. But the pop could be heard from behind the camera. So to give you guys a hint as to where I mounted the camera, the camera mount is a heavy-duty clamp, and I clamped it to the cross sill just behind the center bearing, and the camera pointing down at the connections. On Tuesday I clamped it to the cross sill above the rear u-joint area. This camera takes wide view shots, so it also picks up the space between the bottom of the bed and cab, so you can see all the way up to the transmission when it's facing forward, and the entire axle when pointing backwards.
It's kinda neat to see the underside of the truck while in operation. Definitely a perspective you don't ever get to see.
With that said, and from the videos I have shot thus far, I believe it is coming from the axle. On Tuesday night, it popped, but mildly and you couldn't tell because it happened right at the time of a rough spot in the road. Rough spots don't cause it, it is completely sporadic. The video from lunch yesterday, and last night's video, all indicate the axle is making noise. I owned and daily drove a '65 C20 from 1987 to 2005. It had a 350/SM420 setup. When I got it, it had the HO52, but I found a 1968 Dana 60 and put that in, in 1991. The SM420 is beefy and not quiet. When I bought this truck, it was and still is running the Saginaw 3 Speed, since I had never operated a truck with the 3 speed I figured they were noisy like the SM420. Granted, the transmission needed to be rebuilt when I got it, and while it was being rebuilt I ran a different 3 speed in it and the truck made the same sounds, so I chalked it up to truck transmission gear noise. I may have mentioned it before, but I restate it, this truck has been on many 1,000+ mile trips and not a hiccup. Yes it will pop, but that's it. No loss of fluids, nothing overheats, just the occasional pop.
That said, Tuesday's video had the camera pointing towards the axle and the gear noise was louder than the gear noise with the camera pointing towards the center bearing. The PO did tell me he rebuilt the rear end, so perhaps he did it wrong.
Could it be a broken axle shaft? I don't know, because on tight turns in a parking lot it doesn't do it, the no-spin is locked up pretty good and turning makes the truck lurch not pop. Pushing in the clutch you can here everything finding its happy spot as you finish the turn and turn it straight. Left or right in a parking lot the same, but if using a little throttle it acts normal. Hot Rod Lincoln mentioned something like that earlier in this thread. I prefer to drive slowly in parking lots and let the dummies drive fast and pull out in front of you. Heavy-footed launches, no popping. Just light-footed take-offs and usually after the truck has gotten up to operating temperature.
I have been enjoying this setup, the three-on-the-tree/HO52, but I have the SM420 and Dana 60 still, and have been wanting to install that 420 & D60. So it might be time to get the bearing kit and rebuild it. I have the seals, and other items for it, I have just been enjoying the current setup. The D60 is 3.54 gears, and I do not plan to buy a gear set to re-gear it to 4.57. If it goes in, the 420 has to go in too, or the take-off gear will be too high with my current 3 speeds.
I will take more videos today and tomorrow, all concentrated on the axle and taken from various angles, and I will report back what I see. This turned out to be longer than last night's post, but also has a little more info.