I have noticed a metallic rattle/pinging/vibration coming from the passenger side ex howst. Only happens on startup and then goes away in a minute or two.

What could this be? My first service is in about 6 months. Can it wait to be checked and fixed?

This hasn't happened again, so far. Maybe a valve got stuck temporarily, who knows.

I was thinking maybe a pebble got in there? Or the valve just stuck for some reason.

We had this on our M240iX sometimes.

But i was under the impression that US spec M50 come without the exhaust valve

The car is also basically brand new. Could be heat expansion.