Deciphering Volvo engine noise

[NJC]

My Volvo 4cyl (B230FT) sounds a bit like a diesel Jetta when warm: "clack clack clack" sound. I'm not really worried about it but I wonder what is physically causing this noise.

I've checked for rod knock by removing each injector. The clacking sounds heavier than valves (I have that too, more of a ticking sound). And I thought piston slap mostly went away when warm once alun piston expanded..?

I've ran both 10W30 and 15W40 Castrol with no difference in sound. Car has 125K miles on it.

A bit of history. It was overheated a few yrs ago (enough to warp head) but oil consumption and power are not adversely affected.

Thoughts? I know it's hard to isolate and explain without hearing but anyone have any ideas of what causes a gas engine to emit the clack clack clack of a diesel?

 

[ALS]

Piston slap
The B230's are notorious for this after a hundred thousand miles or so. No big deal since many of these engines will run another 50K to 100K with no problems. It is caused by cylinder wear where the piston rattles in the bore. If the noise gets louder as the rpm's go up than your looking at rod knock (Fatal)
My old B230FT sounded like a diesel toward the end before I put it to sleep due to rod knock.

 

[NJC]

ALS, thanks. I guess I'm unclear as to the underlying mechanical principles.

Why would alum piston, which expands faster than cast iron, rattle more when fully warmed up?

 

[ALS]

Piston rings are made of steel. Even though the piston is made of aluminum you still have the piston rings that hold the piston in place in the bore. Worn rings and a worn bore? Think about it. In most cases it is more a problem with the rings than the engine cylinder itself.
Volvo uses a high nickle iron when it casts the block. In most cases when the engine needs a rebuild it usually ends up with the bore getting honing and new piston rings. That high nickle content means less wear over the long term with the blocks.