OMG it's dead and about to blow up, I'll buy it for $200.00 and save you and your family from disaster!!lol
It's in the valve train: broken spring, broken retainer/guide, bent pushrod, dead lifter.
I changed intake gaskets on a 3100 about 7-8 months ago and it started making noise like that about a week afterwards.. I figured to much coolant got into the oil and it was a rod, but I put my hand on the intake and it felt like something punching back.. LOL Took the valve covers back off to find a guide broke and rod bent. Dunno why, luck of the draw, it warped when I took it off and stressed it when I put it back (since the valve train has to come off of those motors) dunno, but it sounded exactly like that. Take your valve covers off and see if you see/feel anything lose..
