i have a 99 Camaro SS A4 w/ an auburn diff, now my thing is that my 1st and reverse gear have like a clunking sound when moving and gets louder and faster as the car moves quicker but ONLY on 1st gear never does it when in N or 2-4 gear, BUT once it hits 2nd gear and up it goes away and the car drives and sounds like theres nothing wrong with it, its just 1st gear and R... what can it be? and it has transmission fluid and all the gears hook up just fine... anyone know whats wrong or has similar issue?

 

Would that be a bad reaction planetary? Per the GM manual, the planetary (part 681) is only held during 1st and reverse. Perhaps the pinions or the sun gear is going out?

 

That is certainly a possible diagnosis. It could also be the torrington bearing in the rear planetary or between the planetary and rear reaction carrier.
In any case, it will require removal and a rebuild to correct this.