Just picked up a noise in the 300. It's almost like a knock but with a rattle to it. It's not a clearly defined one shot noise, more drawn out. I've only heard it at an idle, got home tonight and heard it when I pulled in the garage. Seems to have a frequency of just over a second, but didn't actually time it. Broke out the stethoscope and couldn't hear anything unusual anywhere I could reach with the probe, but the stethoscope quit working before I played too long. Seems to be most audible when I get my ear down near the right front gap under the intake but I'm not positive it's not reflecting from there. Any ideas on where to start?

Oh, does anyone have a picture of the head with the valve covers off?

Thanks,
Mike