Ok, working on a 1976 bronco. C4, Dana 20 j, Dana 44 factory dics front axle. In 2 wheel drive I don't hear anything and runs smoothly, in 4 hi and low with hubs unlocked runs smoothly also, when you engage the hubs I hear a rhythmic knocking in sync with the throttle ( or vehicle speed). In 2 wheel drive with the hubs locked in runs smoothly, and with the vehicle lifted in the air in 4 wheel drive locked in, I can not get it to reproduce the knocking. Any ideas , I'm starting with the drive shaft I do have a little play in the joint off the TC. Thanks for checking your mind vaults fo me.

That could be a couple of things. Clearly you need load on the system to generate the noise and free-spinning any one part does not re-create the problem.

The next step would be to listen carefully and determine if the noise repeats at a rate that is consistent with axle rotation or driveshaft rotation. Also, does it start immediately when you engage 4x4 or do you have to wait for binding to wind up load on the system?

ALSO, are all four of your tires exactly the same size?

ok, good questions, all four tires are the same, when you press the gas and start rolling it starts knocking, I think it matches up to the drive shaft more then the ring and pin, but I cant get a good feel. It could be hubs or drive shaft right? thanks for the response

If it's synced with the driveshaft speed, it could be an issue with the pinion gear, driveshaft or transfercase gears.

First and easiest thing to check is pulling the driveshaft, flexing each U-joint, checking for gritty-ness or play.

Check your axle shaft u joints in the knuckles.

ditto: if its the drive shaft it would be very rapid. The axles would be a slower knocking (same rotation speed of tires)

OK, pulled the Front drive shaft and had the u joints on the H collar and slip yoke replaced which fixed it. thanks for the thoughts and responses