Speedo Rattling & Jumping

I completed a rebuild of my A-833 transmssion (3 spd w OD) on my 1976 Duster. It was a complete tear down replacing syncro's bearings and seals. Tranny was in good shape, but was leaking from the seals. On my first test drive, when I get up to 20 - 30 miles per hour, I get a rattling noise from the Tranny area and the speedo jumps up to 50 - 60 miles per hour intermittently. I just drained the fluid, and pulled the speedo gear out of the side of Tranny and everything looks fine, no sign of damage etc. BTW it shifts fine and otherwise is smooth, and while I was at it, I changed the speedo cable to a brand new one from Brewers.

Before I put it back together does anyone have any idea what else to check as the source of the noise, and the direct correlation to the speedo jumping?

Thanks