2003 BMW 745 sound in tranny

When I slow down to stop at a sign my transmissions starts its down shift. When it gets to first it sounds like it drops, like a clunk. It happens more often if I approach faster and slow down quicker. I hear it going thru the down shift 3 smooth, 2 smooth, 1 clunk. Almost like first is out of sync. If I approach a stop very slow it does not do it. I am not speeding then coming to a direct stop. Any ideas?

it is a 745I sport, another quick question. I just replaced the tires AGAIN. They seem to wear from the inside out. At about 7000 miles it starts to rumble. The car is aligned. Bmw told me that because the car is so heavy with the sport suspension set up, this is the proper wear for the tires. I find that hard to belive at 350 a tire that BMW would manufactuer a suspension that would gobble tires up so quick. Ur thoughts?