02 monte ss clunking tranny

so when i start the car and drive for awhile, the car runs fine. but after about 15 mins you start to get a clunking noise when the car shifts from 1st to 2nd or 2nd to 3rd, etc. any suggestions?

i dont really want to have the whole thing rebuilt, could it be just flushing the fluids or anything like that?

Mine does the same thing....my is due to 240,000 miles and never had a tranny flush so the valves stick at the shift points and cause clunking...im not going to flush do to never having one its a 50/50 chance the flush will ruin my tranny :-(

Get it checked out, if it is due too a hard shift issue that is internal, but it could be an axle or mount not the trans.