I think chain tensioner is the most common of those.  A plug wire that is shorting out will make a ticking noise also, as can accessory pulleys or tensioners on the belt.  But, it goes away after the car has warmed up??  Try a thicker oil.

As teh spark arcs from teh wire to where ever it is grounding, it will make click or snap nosie as it jumps.

M42's have self adjusting valves so that's probably not the problem, although any oil buildup on the valve train may cause problems with the adjustment, causing them to tick.  This is a known problem with the M20 but you can adjust them yourself, so its easier to deal with. Injectors are also known ot tick excessively and are often the noise you hear.  The association of the sound level with the engine warmth suggests the valves to me since the warmer it gets, the more the valve train will expand and close any gaps that aren't supposed to be there.  The lash settings on the valve train are pretty small, so expansion due to heat will definitely cause the noise to quite down a bit.

cool, good to hear that it's just a minor thing..