Weird Engine Roughness 912ULS

My friend’s RV-12 w/ 912ULS @ 500TT has just developed a slight engine roughness noticeable at just below cruise RPM.  This is really strange…  5500 RPM cruise is very smooth (prop is dynamic balanced), but when pulling back throttle there is a roughness almost immediately.  Carbs are synched very well.  A reduction of just 150 RPM causes a noticeable vibration.  The problem just started yesterday after a 120nm cross-country flight when we began to let down from 5500’ MSL.  Same problem on return flight home.

Any ideas?