Weird rattling noise

[xirdneh]

either a bad bearing on front end of output shaft
its in early stages of getting ready to fall apart
or
something is wrong with the 1st gear/synchro relationship
bad or missing detent key (there are three per synchro) or bad mesh teeth on 1st gear (that mesh with synchro and shift fork slider
could be shift fork too
no matter, the only way to find out is to take it apart
or
could be something simple like the bushing on the end of the shifter
the shifter is the only thing that can be removed without taking the whole tranny apart so its worth a look
remove center console (between seats)
remove boot over shifter base
peel back boot at base of shifter
remove snapring and pull shifter out

[irowiki]

I had a tercel pop out of either 1st or second when going downhill and engine braking.

Two 50/50 flushes of gear lube and ATF more or less fixed it!