Also the exact same situation on my 2011 GSA. At first thought it was right cylinder head based on sound location, but had those adjusted at regular service interval and everything was fine. Currently suspect exhaust flap. Mine happens more in the 4200 to 4700 rpm range, primarily when throttling off. 2nd, 3rd and 4th gears most noticeable. Probably my only single complaint on bike....mentally distracting because I feel like it shouldn't be there. Didn't happen for the first 3,000 miles or so, but has been there ever since. Asked dealer to check it out, they said they couldn't find any issue but I picked up the bike at closing time on a Saturday and everyone was bugging out so I didn't have a chance to drill down on the possible causes. Would be very interested in expert opinions or, in lieu of that, anything comical.