Popping out of gear and/or noise in 1st. and 2nd. gear indicate you've got problems with the reduction gears, countershaft bearings, or other major difficulties. Drop the transmission out and do a full teardown and overhaul. The reason the noise goes away in 3rd. gear is the fact that the power goes straight through without being transmitted through any gears. This takes the load off the cluster gear and the 1st. and 2nd. reduction gears. Jumping out of 1st. is usually caused by a missing tooth on either the 1st. reduction gear or the 1st. gear position of the cluster gear. Don't do any more driving until the trans is repaired- - - -one broken tooth in the wrong place can hand-grenade the whole transmission, case and all.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!