neonflx

I purchased a 99 CLK 430 about a month and a half ago, it runs fine however, sometimes I hear a clanking noise coming from what it seems the transmission, it does it only when I am a very slow speeds coming to a stop (but don't stop) then accelerating I can hear a noise on the the tranny, it does not do it all the time just sometimes, the Benz dealers checked it out and did not find any error codes or low fluids, but the guy said that it could be something internal, anyne has any ideas? thanks