From the RAC:
If there is no misfire or judder when driving and this only happens when idling, possibly a small air leak on the vacuum side of the engine (intake side). Easy test is to spray brake cleaner all over the engine with it idling - NOT the exhaust - and if the engine speeds up for a moment or two then there is a small leak. Then you need to try and pinpoint it using the spray at one component at a time.
Could also be an electrical component such as the crankshaft sensor showing its age but it would need to go on diagnostic equipment to check.
The click could be partial failure of a spark plug or the coil which sits on top of it and that would give the rpm drop too.
2015 2.2 CDTi SENav 163ps manual 4x4 in Carbon Flash with 19\" alloy spare towing a Bailey Unicorn III Madrid @ 1500kg.