Knocking under mild load

[pwjm]

1990 240sx, Bone stock SOHC engine with low miles (<60,000km's)

Under mild load and between 1900 and 2800 rpm the engine makes a bad metalic noise... not quite a ping, but not really a buzz saw rattle. At first I thought it was maybe a bearing or something in the bottom end, but I thought I'd throw some premium in to rule out detonation first. The noise completely went away once I stared using the higher octane gas. Also on cold starts the engine rattles quite badly for a few seconds and it goes away completely, though I've heard this is common on a tired SOHC motor as the timing chain guides wear out or break

I've got a spare motor I'm building up anyways, but it's going to be a while so I've got to make this one last. Any ideas guys?

So far, my to do list involves checking the dizzy to see if it's somehow advanced itself. Beyond that I'm not really sure what else to check. I don't really feel like putting premium fuel in for the rest of this motors life.

[S13_RB25]

Its most likely a chain rattle.

[pwjm]

I agree with the timing chain rattle for the start up, but there's no way running premium fuel would stop the timing chain from rattling while under load... It's got to be something else. I'm actually not even sure how to check the timing on a chain set up... only ever dealt with belts before.

What can cause detonation on a stock motor?

carbon buildup on the piston/valves ?
Timing somehow too advanced?

[S13_RB25]

The rattle goes away for a while, same thing happened to me with my single cam. But take the valve cover off and look down, where the chain is, cause one of the guides is made out off plastic that breaks easily. Also my rattle is gone now too and I'm also running premium fuel, 93 is the highest they have here on pumps.