Engine Knock at Idle and Cruise

This is a Ford 390 (?) in my grandfather's Ranchero. I posted it here because it could apply to any make/model engine I suppose.

He says that when his motor gets up to temp, there is a slight knocking noise from somewhere around the #8 cylinder (rear, pass) at idle (more noticeable) and also when cruising (less noticeable but getting worse). It goes away when accelerating and decellerating. He had someone listen with a steth and couldnt determine whether it was upper or lower end but it is definitely there in that part of the block.

Does this sound like a wrist pin or rod bearing? Cracked piston? What would be the best/next course of action?

any suggestions would be appreciated.:)

Thanks Denny.

I havent heard it either. This info was based on his description. I'll pass it on (or go listen to it myself) and check back with the results.

Denny pretty much covered it, except for one hint. If the knock is a scored piston or wall, you can tell by the acrid, burnt smell of the exhaust. I don't know why it smells like that, but they always do.