There are a lot of posts on the K1600 Forum from riders complaining about a "drive line lash" or "clunk" when shifting. They are simply not shifting properly. When I first got mine I discovered that you have to shift with very little clutch disengagement and very little throttle movement. If you fully close the throttle and pull the clutch lever fully in you will experience a rough shift. However, if you barely move the clutch lever and the throttle, all you notice is a change in the tone of the engine as it accelerates. It is by far the smoothest bike I have ever ridden...and I have been on a lot of them in 46 years of riding.
Tommy Crawford
2012 1600GTL
2012 1200GS
2010 Harley Ultra Classic Limited