Why it happens on startup specifically
The timing chain tensioner relies on oil pressure to keep the chain correctly tensioned. When an engine has been sitting for a while, oil drains back to the sump, so the tensioner takes a moment to build pressure again after start-up. If the tensioner is worn, this brief delay is enough for the chain to slap against its guides, producing the rattle. As wear progresses, the noise typically lasts longer after each start and can eventually become present at idle too.
Is it just the rattle, or is something else going on?
- Rattle lasting more than a second or two on cold start-up
- Rattle that's become more frequent or lasts longer over time
- Any accompanying warning light or rough running
- Noise at idle, not just on start-up — this suggests more advanced wear
Should I worry about it?
Yes, in the sense that it's worth having checked promptly rather than hoping it goes away. A rattle is the chain telling you it has slack — and slack can, in time, lead to the chain jumping timing or, in the worst case, snapping. Caught early, this is a straightforward, fixed-price repair. Left too long, it can turn into a full engine rebuild.