Well, still a mystery to me at this point. Daughter's pick up. 5.2 - NV3500 - 4 X 4. Thanks in advance for any input or clues to find this problem.

Problem: A loud grinding / rattling noise coming from the transmission area beginning right before the shift point. Truck has no tach, just shift light, typical shifts done above the shift light coming on. Happens most pronounced at the 1-2 and 2-3 shift points, but sometimes at the 3-4 and 4-5 shifts.

My trouble shooting started with the rear axle on jack stands with an assistant running it through the gears while underneath with a long screw driver (hillbilly stethoscope) to my ear. Noise seemed to be coming from the front area of the transmission. At 180k I think, input shaft bearing going bad in the transmission. Since it also had a problem jumping out of first gear, I decided to rebuild the trans. Didn't find any bad bearings in the process. Replaced input /output bearings plus counter shaft bearings, input to output bearing, synchros, 5/R shift fork, plus 1/2 and 5/R syncro sliders. Engine had been swapped just before purchase about 5k miles before by previous owner who stated a new clutch was installed at that time.I visually inspected the clutch without dis-assembly and could see the flywheel had been surfaced, and fresh appearing pressure plate and clutch disk. The throw out bearing also appeared new and was smooth and free turning.

Got it all back together last night. The good news is, it no longer jumps out of 1st. Huge disappointment to learn the noise is still there. Additional test driving last night revealed the following;
1. Very light on the throttle easing up to much higher shift point than normal, the noise never happens on the 1-2 or 2-3 shifts. Not tried above this.
2. When the noise comes on, clutching with throttle still on to keep rpm's up, letting off throttle without clutching, or normal shift action (off throttle, on clutch), the noise stops immediately.
3. Sitting still, in neutral with clutch pressed or not, and engine rev'ed well past point noise is heard when driving, the grinding / rattle never happens. Load related, it seems.
4. Weird twist 1: When the noise comes on, if I just stayed on the throttle, the engine fell off as though I had let off. While holding throttle position, the noise abated as it slowed until a point when the engine came back and began to accelerate again, at which point rpm's would climb and the noise came back. It repeated this in 1st and 2nd.
5. Weird twist 2: On repeated take off's in 1st and heavy throttle, when it would grind / rattle, I let off then almost immediately mashed the throttle again. On two of these instances, it then wound out with a shift to second without ever making the noise.

Checked codes this morning. None.