Based on the sound, the problem was the intermediate drive gear coming loose in the water transmission. I had it happen twice years ago until I replaced the retaining clip on that intermediate gear shaft. It had evidently become sprung enough that even though I wouldn't think it would have much force pushing it off, when I replaced that clip, that fixed the problem, and it's been many years since I had the problem with only one of my cars. Unfortunately, this problem usually ruins the top hat (shaft) and damages the inner surface of the gear, so that gear should be changed too. It can be done with the transmission in the car but it's a mess since you end up dumping gear oil from both trannys when you pull the water tranny cover.
last year, i did new water trans seals and bearings, and new c-clips on the drive gears
But dammit - i cant recall if i did 2 things
1) torqueing the center drive gear nut to the suggested 86 ftlb
2) bending up the "tab" to secure the nut on the center drive gea