My ride is a 2010 Specialized Roubaix; with 105 FD and RD. The rear cassette is an Ultegra 11-28; front rings are mid-compact 52-36. These latter components, along with the chain and cables- are fairly new -- maybe 1000 miles on them. FD and RD are original with 7-8K miles. RD was last adjusted maybe 500 or so miles ago. Ride 100-125 miles a week, with a varied mix of hills/flats/rollers etc.

Lately, rear cassette starts to chatter when in the small ring front, and the 2d or 3d largest cogs on the rear. No chatter on the largest cog or the smaller cogs. No chatter when on the big ring, either - only when on small ring. Shifts are crisp up and down the cassette.

Tinkered with the cable tension, using the barrel adjuster, to no avail other than sharpening up the shifts a bit. I figured that the cable had lost tension since its last tune-up, so I tightened up the cable tension using the barrel adjuster, which helped. But I can't resolve the chatter. Thinking maybe the B-screw(?). Seems that the cogs on the cassette are hitting the pulley on the RD when on those 2 cogs.

thoughts?