Cassette jerks chain forward intermittently I have a 3 year old road bike with Shimano Ultegra 11 speed freehub CX-75 and cassette CS-6800. Recently I started hearing clunking noise from the drivetrain during freewheeling. It appears that the cassette turns forward along with the wheel for a small angle, pulls the chain with it (this squeezes rear derailleur and creates slack on the upper chain run, since the crank is not moving), then releases, upon which the derailleur springs out quickly with the clunk.

I can almost reproduce the problem at will. It happens more often in cornering. And the chain does make a clunk or two if I wobble the handlebar quickly a few times when going straight. The common denominator is a sideways acceleration, but whether the problem is triggered by a little frame deformation under side load, or the inertial force on derailleur or other components, I cannot understand. Clunking does not happen when I set the bike hanging on a stand.

I could not also find any description matching the symptom. Should I replace the freehub? It must not be worn at 3 years, and probably way under 10K miles, Ultegra being pretty higher end set. But I cannot think of anything else.
Since the cassette is being driven by the wheel, the freehub mechanism is catching instead of releasing.  

Confirm that by removing the rear wheel, remove the cassette, and spin the wheel/freehub off the bike (ie in your hands)

When powering it through the freehub the wheel should turn like normal, but as soon as the freehub is not driving the wheel, it should stop and will make a faint or loud clicking sound.    If you feel the freehub still being pushed around by the wheel, it needs work.

Fix0 - clean all the impacted dirt and oil, hair and grass out from behind the cassette and around the freehub.   Consider removing the dork-disk if its cracked or brittle or not sitting right.

Fix1 - blast the whole thing with cleaners, then let it dry, and lube it up again before use.

Fix2 - disassemble the freehub and attempt an interior service.  

Fix3 - replace the freehub mechanism (last resort)