I have a 98 Z28. My question involves the drivetrain. I have a stock automatic trans and in the rear I had a auburn pro series posi installed with 3.73 motive gears. The gears and posi have been in the car for about 2 years with no problems at all. The transmission has been tuned through the PCM with an HP tuners program for firmer shifting. In the mornings when the transmission is cold and shifting a little firmer Ill hear a clunk noise under my car when it shifts from 1st to 2nd. Would this be a trans mount? Or should I be concerned about bigger problems? I makes no noise when I first put it in gear. Occasionally Ill here the noise on take off but mostly when it shifts from 1st to 2nd. Thanks

 

I would inspect the mounts and torque arm bushings as the most likley source,