clunky noise 1st, 2nd gear

Have a manual trans and there is a clunk noise when depressing clutch going from 1st to 2nd and depressing clutch to go back to 1st at low rpm maybe 1k-2k in parking lot situations at low speeds 10-15 mph, where you could probably remain in 1st gear. The noise only occurs after the engine has warmed up, 20 mins or so
It doesn't happen on street driving say starting out at the green lite to get to the speed imit shifting at higher rpms 2k-3k but just when you're creeping through a parking lot. I've got an extended warranty thru next april and will take to dealer unless it's just a normal sound in low speed situations. Anyone experienced this? Thanks.