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.
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