Strange engine knock

I have a 2001 chevy venture 3.4L. I am getting a strange engine knocking sound, very loud and it keeps in time with the engine speed. The strange thing about it is the fact that it is not a constant, steady knock like you would expect to hear from a rod or collapsed lifter. at times it will stop completely for a second and then start again. It sounds like a metal on metal knock. I have removed the belt to rule out something in the accessory drive. I have also removed both valve covers and inspected the rockers at all 360 degrees of cam rotation. There are no loose rockers. I am guessing it would be a rod, but wouldn't it be a constant knock? I have never heard a rod knock that went away at times. Any ideas would be appreciated.

Jim

You are using regular 87 octane fuel? That's what is recommended for the 3.4L.
Knock sensor fault? Probably not.

You may need to try and locate the knock with a mechanic's stethoscope.

The only thing I can think of is the well known piston slap in these engines, but that usually quietens down when warmed up.

Perhaps one of the techs on this board will have a better idea.

A noise that comes and goes sometimes is related to a shaft/pin that moves. My guess is crank or cam end play (not likely) , but more likely is a wrist pin.

a couple of questions to help:

Is it worse when engine is cold?
how long has it been knocking?
did it start gradual?
is it worse at high rpm?
does it do at idle?
which end of engine does it seem?

questions really just to help process and diagnose.

To answer questions...
-Knock is the same whether engine is cold or warm.
-Started knocking a couple months ago slightly, now it is much louder.
-It got loud immediately though, it was not gradual.
-it is not worse at high rpm. it is consistent with engine speed but does not get worse.
-it does it at idle.
-using a mechanisc stethiscope, it is much louder at the front valve cover than anywhere else.. But I have inspected the valvetrain for loose rockers and there aren't any.

My money is still on a wrist pin right now. I generally let these noises go while paying close attention to circumstances when it disappears and such. Eventually if you keep tinkering you will find it. I'm not saying to not fix it. A couple of pins that walked put two grooves in the bores on me. I wish I'd not let that one go.