A single-VANOS E36 (M50TU and M52) that clatters on a cold start — a marbles-in-a-can or light-diesel rattle, loudest around 1,000-3,000 rpm and fading as it warms — has a worn VANOS, and it's not the emergency the noise suggests.
Single VANOS is a simple unit; the rattle comes from the anti-rattle spring and tired seals, the seal kit is cheap, and cars run for years on it. Alongside the noise you'll notice a slightly rough cold idle and a softer mid-range.
Fix it when convenient rather than treating it as urgent — but don't ignore a rattle that's clearly getting worse.
This is a different, far milder thing than the E46 M3's dual-VANOS trouble, so don't let S54 horror stories scare you off an otherwise healthy M52.