E61 520d cold start tapping noise

[GR4]

Title says it all really. 2005 car, now done 58k miles. Has anyone else experienced a quite pronounced tapping noise for first couple of minutes after cold start. Can't hear it inside car, but if you stand at front with engine running it is quite clear. If it is a hydraulic tappet taking a while to fill, is it fairly common and harmless, or should I be worried ? Car still just about under warranty, but of course it is always warm when I get to dealer so does not do it. Any thoughts appreciated.