Rattle from tranny in 3 and 4th

[grannyknot]

Just got the car on the road recently with a new to me close ratio 5 spd tranny.

In all ways I would say it works very well except in 3rd and 4th gear on hard acceleration there is a high pitched rattling from the trans that I can hear and feel in the car and can hear coming from under the car through the windows. Does this mean a rebuild is needed? I'm running Redline MTL 75/90 trans fluid.

Thanks,

Chris