Gentle tapping noise from idling cold engine.
 

I've noticed this before, so it's not new, but I started the car today after a couple of weeks non-use and got out to clear debris from a wiper and could hear it.
Cold, wet, miserable day so I didn't open the bonnet to investigate/locate.

Regular very light tap, not harsh more of a quiet duh, duh, duh noise roughly 3 to 4 times a second with the engine doing a fast cold idle. Could have been a belt or bearing or anything rotating I guess.

At that frequency, any thoughts what it might be and whether to worry?
Can't hear it in the car with the doors shut and IIRC it's not there when warmed up.

Injector?

Probably too slow for an injector (or valve?). At say 800 rpm a 4-stroke engine will be firing each cylinder 400/60 times per second, i.e. about 7.
It's not that rapid as I can count them while watching the second hand sweep on my watch.

Hamster with a wooden leg?

I've certainly had a ticking sound when cold - a very discrete, sharp "tick" - which after running a few minutes goes away. Usually apparent after periods of non-use so its not always there.

Also, if its idling at 800rpm (13.3/sec) then the 4th (firing) stroke is around 3.25 per sec which was about the rate of my ticking. But CAN an injector "tick"?? I dont know - just thinking out loud, here!

If you take the engine cover off you can clearly hear the injectors ticking as they fire. Its not impossible for one to get louder than the others - you would hear a difference in the pattern.

A tick that goes away quite quickly is usually a hydraulic tappet. As they age they leak more static pressure when the engine is off, and can take a little running to warm up the oil to pump up fully. Not a problem if they quieten down quickly.

I've found they don't particularly like being parked nose-down either, for some reason, even thought that is the best attitude to prevent water ingress. My S8 can make quite a rattle and a cloud of blue smoke on cold-start when she's been sitting nose-down in the front of the workshop. Parked the other way round or flat she doesn't do it at all.

Thanks MJ, that's probably it, I'm used to the noise of hydraulic tappets on older cars (especially Rover 800's of which I had quite a lot!).

I am parked in a slight nose-down attitude and do also often get a bit of blue smoke on startup, though overall oil consumption is zero!