"The noise in the M102 definitely sounds like the regular tick-tick of a lifter to me. Its repetition rate is directly proportional engine speed. Only comes and goes during warm-up. Once hot it never appears. Should I worry or not? How would I determine which rocker is the noisy one anyway?"
The thing about a noisy 'rocker arm' is that it is not the only thing that can stick. If the valve is sticking, you will get exactly the same noise. Since it happens consistently when the engine is cold, perhaps a compression check will differentiate, and also narrow it down to one of two valves, if that is the cause. If a rocker arm, I would think you could tell by feeling the action and clearances with the valve cover off.