A rattle, tick, or tap that's loudest in the first second or two after starting a cold engine — then quiets down or disappears as the engine warms up — is one of the most commonly searched car noises, and for good reason: it's the earliest warning sign on several of the most-discussed engine issues on this entire site.
This noise happens because oil hasn't yet built up pressure in a component that depends on it — most often a timing chain tensioner, a variable valve timing (VANOS) actuator, or a chain guide. While the engine is cold and oil is thick and slow to circulate, there's a brief window where that component isn't fully supported, and it rattles against its housing until oil pressure catches up.
On some engines, this is a brief, intentional characteristic that doesn't indicate anything wrong. On others, it's the first audible sign of wear that will progressively worsen and, in the most serious documented cases, can lead to the chain skipping a tooth or failing outright — which causes catastrophic internal engine damage. The engine matters far more than the noise itself in judging how worried to be.
This symptom shows up across several unrelated engines and drivetrains — each has its own root cause and its own full writeup.
Identify the exact engine first — the same noise means something completely different depending on whether it's a BMW N47 (potentially catastrophic) or an Acura K24 (a cheap solenoid fix). Don't evaluate the noise in isolation from the specific engine.
If you're the one hearing this on a car you already own, get a specialist diagnostic rather than waiting to see if it goes away — on the more serious engines in the list above, waiting is exactly how a cheap fix becomes an expensive one.
If you're evaluating a used car with this noise, treat it as a negotiating point at minimum, and on several of the engines above, as a reason to walk away without a documented repair history.