i've had this drill for years and by the looks of the guts, it will last me a lot of more years to come (looks like it's built to last)... love the belt hook, BTW.

although it is not the same model, it basically the same innards like this schematics show...
http://www.mendingshed.com/dwd110k-type-1.htmlbut yes, the "clunk" (like hitting wood very slowly with your knuckle once) is in my drill too... i have to pull the trigger really quick to get it though... and it does not do it all of the time... it never got my attention until i read this.
but you might be right regarding the gears "loose". if i move the chuck's big gear, there is some play there... now i don't know if it is due to the rotor twist gear design that causes that, but my dollar is that the wiggle room is what is causing the "clunk" you are experiencing... would that make me sell my drill? no (like i said, that has never even bothered me before). but i am not you, so, if the "clunk" bothers you, i would get my money back. IMO.
BTW, cordless is another animal... don't compare corded with cordless.
my Milwaukee cordless doesn't do this
also, i have other corded drills at work, i may take a peek inside when i have time.