Just to satisfy my own curiosity, I hope there is a board member on here that can tell us what exactly this noise is. A had a 1995 Civic with a d16 that made an awful noise when cold but was dead silent after it was up to temp. I adjusted the valves and ruled them out. I knew it wasnt valves anyway because this noise was only UNDER LOAD. I just always let it warm up several minutes and never had any issues. I sold it to a buddy for cheap and he drove it for years with never an issue but the noise was always there. Dead silent at normal operating temp. I know it has to be piston slap or rod bearings on the high end of tolerance but never knew for sure. It sure wasn't worth tearing into just for curiosity. This was a 1995 d16 SOHC. D16Z6 if memory serves...
It is piston slap and is due to short skirts and excessive piston to bore clearance that gets taken up when things expand as they get heat into them.
Rod knock happens when the oil thins, ergo, you won't get rod knock on startup as that's when the oil is the thickest. An engine with excessive rod to journal clearances is usually quiet on start-up and gets progressively louder as it warms and the oil thins.
In comparison piston slap is most pronounced when an engine is stone cold and will get progressively quieter, usually going away completely as the engine gets up to temperature.
They can sound very similar but how they manifest is completely opposite.