1994 k2500 Suburban with 6.5l, I’m kind of new to diesels and I thought when I got this truck it would be a great way to learn diesels, boy was I right owned 3 years and had to replace many different parts and diagnose many different things but I’m stumped now.

Problem:
I was driving down the road around 50-60mph when I came to my turn so I left off the throttle and then this G-d awful sound came from the engine bay but no abnormal vibration. Luckily there was a parking lot I was able to pull into and shut the engine off and check a few things out. Gauges were all normal, no fluid omg leaking onto the ground or anywhere I could see. Started it back up and same noise. Shut it off unhooked the PMD, and started it same noise. This is where it gets weird, started it back up and went down the road a little to the gas station and the noise went away after going above 35-40 mph and above 2000 rpm, let off the throttle noise comes back. Best way I could describe the noise is a really bad rod knock but based off my knowledge of vehicles it’s not a bent or broken rod.

What I’ve tried:
Replaced all 8 injectors, and glow plugs, checked flex plate bolts, took the belt off and ran the engine with the belt off to rule out an accessory. No change. I’m completely stumped

Could it be a valve or lifter stuck? If so, is there a way to test before tearing into the engine. Thanks for any help.