BMW X3 B58 cold start rattle: what is normal chain slack, and when to actually worry

A brief metallic rattle for the first second of a cold start on a B58 is normal chain tensioner take-up, not the failure pattern the older N20/N54 chains had.

The B58's chain and guides sit at the front of the block, in easier reach than the N20's rear-mounted setup, and the failure pattern that made the older engines notorious has not repeated on the B58 in significant numbers. A rattle that persists past a second or returns at idle once warm is the one worth a proper inspection.