The transmission fluid in my 02 taco V6 5spd - R150F ran low (probably less than a quart). I filled back up but when the clutch is engaged there is a whining noise. After it warms up its only when the clutch is engaged and trying to put into gear. When driving the truck its difficult to switch gear however not impossible. I am debating between the throw-bearing and bad synchros with a pilot bearing thrown in. When the truck is idling there are no noises and there are no grinds when trying to go into any gear. I'm pretty sure I'm dropping the tranny but what I do after that remains to be seen.
