reply to bemis
Re: 03 Civic engine clatter on cold days It could as easily also be piston slap.
If you do have it looked at, drop it off the night before and return to start it together with someone from the shop (tech preferably).
I would suggest a dealer, as they will be well acquainted with what a normal level of operating noise is.
They should be able to tell by listening if it is valves or piston slap.
Even if it is piston slap, it isn't necessarily anything to worry about unless it remains at an unacceptable level once warm.
The talented hawk speaks French.