69 b-body 4 speed grinding while in 4th

I posted this in the race section, some suggested that i post here.
fresh 440/505 and I rebuilt 4 speed probably in 2007 or 2009.

all has been fine until I recently hammered thru the gears going up the highway. After letting out of it and cruising along about 70 (ish) the trans made a loud grinding noise and I had to pull over. (sounded like gears meshing)
after pulling over and manually shifting car while engine off, i restarted it and limped down to the next exit in 1st gear.
Car is fine around town at low speeds, just does this same grinding after hi speed in 4th gear.
I pulled the trans out (23 spline) and took it apart and see nothing obvious.
I looked in the scattershield at the crank/ bronze bushing and see nothing obvious.
I am using a centerforce dual friction clutch.

Has anyone seen this before? I called Wayne at Brewers and he has never heard of this.I am thinking of driving it up to him for a look but would rather get help on here if at all possible.
I can upload pics of the internals but really all looks well except the cluster gear washers cause very slight play, fron output shaft has play( i know this is common due to the nature of the front bearing)and tail shaft has very minimal play. the countershaft keyway has not slipped.

thanks in advance.

John

Do you have a spare tranny you can put in to make sure its the tranny?

I replied to this in the race section. I still think it is a main bearing, or roller bearing issue. If this tranny set for 10 years with no lubrication and not being rotated occasionally, there is a good chance that the roller bearings have some pitting.. Installing a spare tranny (as suggested) would tell you allot. I have a spare 23 spline, but I'm guessing you're kind of far from where I live..

I'm sorry I mis communicated. It didn't mean it sat,I was only saying I rebuilt it back then. The car has only sat for a year

So, the car was running with this tranny and having no problems up until 2 years ago. Then the car sat for one year and now it has a grinding (growling) noise when in 4th gear. Have I got this right? Does the noise oscillate? Does it come in at a certain speed then go away to come back in at another speed? Does it start as a low pitch then get louder as speed increases??

Why don't you clean up the spare tranny and pop it in? Its not that much work - is it? Yours is a 23 spline - correct?

I know the spare sat for a long time, but the garage it was in is a good climate. I would take the chance.

I would pop the cover and inspect it and if looks good flush it out and refill with correct oil.

Why don't you clean up the spare tranny and pop it in? Its not that much work - is it? Yours is a 23 spline - correct?

Mark, because I would really like to find the reason mine is making this noise- there's not a whole lot to these transmissions and I'm still scratching my head.
I'm one of those people that need to know what happened,lol!
I showed it to Dan @ Brewers over the weekend: aside from the cluster washers being junk and the ends of the cluster gear needing machined, it all looks good.
He said there was some debris that has gone through it.
I'm sure this is a result of the shift fork breaking a year or so after I rebuilt it.
I'm gonna pull the scattershield and see if maybe a spring broke on the clutch disc.