Noises During Shifting problems of the 2008 Chevrolet Silverado

I have a 2008 Chevrolet Silverado crew cab that makes clanking sound when the transmission shifts into third gear. I went to the local dealership and was told that this is "normal for this truck". The truck has less than 16,000 miles on it. The mechanic told me that he was "surprised that it had not started making this noise sooner". It has become progressively louder and can be heard with the windows down. Coincidentally, this is the same sound the truck started making a few months ago when I would take off at a stoplight/stop sign. I could feel a pause and then it would clank and start to move. Again this can be heard with the windows down. I went to the dealership at that time and was told "this is normal operation". Now the truck clanks when taking off from a stop and when it shifts into third gear.