If the Di2 rear derailleur of the ST2S or ST5 grinds, creaks, rattles or the chain jumps, it may be due to the setting.
See 'Di2 rear derailleur adjustment'
Often, however, it is simply due to wear and tear or a lack of maintenance.
See PROBLEMS – Chain (Sprocket) / Pulleys and Belt
- 'Chain jumps, slips, crunches, is thrown off'
- 'Chainring / bottom bracket - wear and tear?