I have this noise/rattle I can't find.

I'm beginning to fear it's piston slap. Can some one give me a good way, without pulling the motor, to diagnose this?

It kind of sounds like detonation but it isn't, I hardly ever hear it when the motor is under a load.

If I put the trans in 3rd and hold the brakes hard to really load the engine (manual VB) it doesn't make the sound.

You'll hear it while cruising easy, sometimes on decel and occasionally at idle.

Another way to describe the sound is it sounds like a bolt rattling around in the exhaust pipe or like a loose bolt where the collector adapters bolt to the headers (would be nice but that's not it either).

Here is a thought,,,

Can you make it do it consistantly? If so, grab a insulated plyers, and individually remove plug wires as its idle/running, this might pin point the cylinder/cylinders with the issue. I have heard worn rocker arms make a similar noise. Do you run the good ol trusty stamped steel OEM rocker arm? I had a 318 that would rattle at idle, and sometimes under load if you knew how to do it. You can take an old valve cover, cut the top off, install it so you dont get oil everywhere, and put slight preasure on each rocker with your thumb. AT IDLE ONLY!!!, and you might be able to make the noise go away.

Kasey

Loose converter bolts?

Not converter bolts and no I can not make it do it consistently, this is part of the reason this has been such a pita to locate. The only time it seems to do it when idling is in the middle of traffic with a zillion cars around, never can make it happen in my driveway.

I drove it tonight and never heard it until I was almost home, after about 1/2 of driving.

Sometimes I'll hear it within a minute of leaving the house, no rhyme or reason as to when the sound appears.

As far as rockers, I have what ever rocker Indy sells for SR heads and a solid cam. it's not coming from the valvetrain, I can almost guarantee...

A friend has a engine that upon firing for me was making a good bit of rattle...It was cool out and the engine was cold. This was a low deck short rod engine BTW. After warming it wasnt near as bad...Im pretty sure it was piston slap we was listening too. It was just noisy, kind of like a 24V cummins in the morning...by the time you get to town its not near as bad as it was.

Well mine is a low deck stroker with 6.700 rods, I don't think these are considered short

But like I said the noise will occur sometimes when the engine is cold and other times you won't hear it for a long time.

Sometimes when you hear it, it only lasts a second or two and then it might be a while before you hear it again if at all, or it may rattle for a couple minutes on and off. It's never predictable, so I always figured it must be something other than the motor doing it, it sounds like something rattling on an exhaust pipe but I can find nothing touching or close to touching.

This is the second summer I've been searching for it. I thought I'd found it when I discovered my damper was bad, replaced that at the beginning of the spring but that wasn't it obviously.

Being as it's a "pedal car" have you checked end play on the crank?

Randy

Oh! them mods! To check end play in the car you just need to pry behind the flywheel and/or damper for any noticable motion. Seeing as how most engines are assembled with less than 0.008" end play if you can see any motion at all you have a thrust face problem.

Randy