Ok.... not exactly an f-150, but its still a ford

1975 Ford Econoline E-150 van, 351 motor, 3 speed manual transmission.

About a month ago I had to put a new transmission... and by new I mean I got one out of a junkyard.

For about a week (maybe 100 miles) I had no problems. Then I started getting a whine while driving. The whine is loudest at slow, starting to move, speeds. The sound is very noticible inside and outside the van.

Any ideas? Could this be caused by not enough gear oil in the transmission (I think it may be leaking out of the front seal)?

Thanks,
Sal