trans issue???

I have a 01 sunfire gt auto with 41000km

The car runs and drives great, however, when i come to a stop at a traffic light, 5-6 seconds later, i hear a clunk in the transmission area and the RPM's drop a tad.....this is either in park, or with my foot on the brake in drive...sometimes it happens 2-3 times while at the traffic light

any ideas? thanks