When it comes to cars, the unexpected is usually bad and changes in state are almost always for the worse. The following are some of the more common sounds made by a worn engine at start up. Some are more serious than others, but any noise should be investigated, particularly if it gets louder or faster with rpm.
Generally, an engine will make a consistent tapping sound if some part of it is not receiving oil at start-up.
Some components that will make noise with low oil pressure are the lifters, valve lash adjusters, rocker arms and variable valve-train adjuster (if so equipped.)
Some components like piston rings will wear over time and must expand with engine heat before they will work properly. Modified engines with forged pistons will do the same, but this is normal.
This problem is more common on some engines than others, but is a result of wear on the valve lash adjusters, valves, valve seats, and/or valve springs.
Older engines with an external exhaust bypass can rattle at start-up, as the soldering used on these tubes is fragile and breaks over time.
Some newer engines inhale a bit of air on cold start-up to check for evaporation in the engine. This is nothing to worry about, though it does get a bit louder with time.
