[kawagumby]

I forgot to include that engine timing affects the intensity of the rapping noise...louder with more advance. Sorry 'bout that.

So, it isn't drive train related and really acts like a rod bearing with too much clearance, but why won't it sound off enough to hear it during rev-up-down or idle? And why does it get so increasingly loud when braking in neutral only to fall back to quiet once the car stops?

Crank bearing sounds (in my experience) are duller and of a lower tone that this noise...which is rod-bearingish. I wonder if I'll just have to pull the pan and feel for excessive up-down rod-play or plasti-gauge each one. Can oil flowing to the front of the pan under braking be a reason for a looser rear rod to get louder?