Vehicle; 04 Taco 2.7L 5 speed, 675000KM's

So its had an odd vibration/noise for about 4-5 years. I first noticed it after replacing the clutch at about 585000KM, so if it had any thing to do with the clutch replacement my guess is that it would of "blown up" by now! I really don't associate it with the replacement but simply noticed it afterward. The replacement went smooth and without incident or anything unusual. OEM clutch and the flywheel was faced. I'd say less then .015" was removed.

The noise/vibration sounds like an engine labouring sound but with a distinct metallic raping sound. It only occurs around 1700RPM in 4th gear but is totally absent below that and above that. That engine speed roughly equates to 55-60KPH (35-40mph). If you shift into fifth it doesn't do it at that road speed it...kind of kills the labouring engine theory. It doesn't seem to be there in first, barely noticable in second and third, very evident in forth, and completely absent in fifth regardless of the road speed. It does have that loose/rattling heat shield sound but read on.

All other speed and RPM ranges the truck runs as smooth and quite as you would expect from a 19 year old truck with that much use, perhaps even better from other vehicles I have been in/drove. in all other speed/RPM ranges the truck runs smooth and quite. At highway speeds it is silky smooth.

I have checked all the U joints, the slip yoke, centre bearing, output shaft of the transfer case and the pinion of the rear diff. I only found a touch of play in one U joint (all original) and in the centre bearing. I replaced with no change. I changed the transmission/transfer/diff oils at about 625000KM and did not notice any abnormal contamination...drained the oil through a filter to inspect for "chunks" right out of the truck. Refilled with a different brand then was in it with no change (75W90); however about 10000KM later I rechecked the oil levels and the rear diff was a bit overfilled so I let it drain to the exact level and the situation had a very minor improvement. I checked the engine/transfer case mounts and they are good. When doing the clutch nothing abnormal was noted with the input shaft/bearing; no alarming sounds were noted when turning the transmission shafts by hand with it out of the truck. I do notice that when it is cold, freezing temperatures, it does get a bit worse. I have also checked for loose exhaust, heat shields, body parts etc. The noise seems to originate from the front of the passenger side footwell/firewall area. It has not got any worse from when I originally took note of it.

Now here's the real head scratcher...this is completely absent if the truck is operated in 4WD. So it seems its an engine/transmission issue when operated in 2 HI, but then in 4 HI it seems to point to a rear drive train issue. (Strain on moving the truck is spread out to another drive axle) I know it sounds like a clutch installation/transmission input issue but it doesn't do it in 4WD in all gears ( Hi or Low range) and absolutely for sure not in 5th in 2WD. Subsequently I have also done those leaking rear axle seals, new bearings, bearing housings, seals even backing plates as they were rusted out...no change. I spun the diff with the half shafts out and the driveshaft disconnected and noted nothing unusual. I have checked the driveshaft "timing" and balance and have rotated it into different positions on the transfer/diff flanges without improvement.

It was suggested the "element" in one of the cats may be loose. They are the originals. Perhaps they got hit while doing the clutch? I can't remember if that happened.

I thought it may have something to do with axle wrap-up when accelerating as it happens during more then about 1/3 throttle being applied but that would point towards items I have already looked into.