Hard upshifting on 05 lacrosse

with 65k my lacrosse just started 'clunking' just after upshifting to the next gear. this doesn't happen all the time, seems like when engine/trans gets hot especially during my hour commute in stop & go traffic, but in the cool morning not at all. Any ideas what i can expect the auto repair shop to tell me after i drop it off !

The clunk could be internal in the trans caused by a stuck pwm solenoid or something in the drive train, from here no way to tell.