I have a Planet X EC-130 carbon road bike with Ultegra group set. I have the stated problem above; it mainly happens while in the smallest chainring and does it at all speeds. It sometimes goes 20 miles with no issues then will do it 6-12 times in quick succession.
I have had a friend ride the bike while I observe and the rear derailleur is being snagged somehow and springs towards the front of the bike before returning to its normal position, it doesn’t cause any issues just a very annoying loud metallic sound. Rear cassette and chainrings are fine, derailleurs have been thoroughly cleaned, new chain has been fitted and bike is kept and maintained to a high standard.