Could it be the 'Slack Tappet Simulator' (STS) as I call it?
Click:
https://the75andztclub.co.uk/forum/s...rchid=17845112
Contrary to RAVE, the STS often occurs within seconds of a cold start (especially when the engine's not been run for a while and the fuel vapour cannister is saturated). The tapping follows the engine revs and will persist for quite a while. It's part of the emissions control system and and is quite harmless.
TC