When it starts from cold, after being stood for at least 2 days, how quickly does the engine quieten down ?
I should take no more that 2 seconds for the tappets to pressurise and for the cam chain tensioner to take up the slack.
Have someone start it for you and have your head under the bonnet !
If it is longer then either the oil is too thick when cold or the oil pump may be on the way out or the pickup strainer might be partially blocked.

I have had success, on two older cars now, of using 5W40 fully synthetic to shorten this "quieting" period.