I have a 1996 Nissan Pickup that I had towed to the shop because it made a horrible gear grinding noise when I let out the clutch while in gear. The shop just found almost an inch of metal shavings inside my transmission. About one month ago I had the exhaust replaced on the truck (at a different shop), following which there was a whining sound when I accelerated in 3rd gear. The shop told me this was the differential and I shouldn’t worry. Could they have been wrong, and this was a symptom of the transmission problem?
Was there transmission oil still in the transmission? Is this transmission a four speed or five speed? The whining in 3rd is an indication that either 3rd main or counter gear were damaged or the input shaft bearing was in trouble. I assume you are getting grinding in all gears when the transmission failed and when you engaged the clutch.
If so what probably happened is that the input shaft bearing failed; allowed the input gear to unmesh on the counter gear; striping off the tops of the teeth of both gears; and depositing the shavings into the oil taking out everything else.
The only way to determine if ‘the shop’ was wrong is to install a new/rebuilt/used transmission and see if the whine is gone. If there was sufficient oil in the transmission, there would have been nothing else ‘the shop’ could have done to forestall destruction of the transmission.
Hope this helps.