Chevrolet Repair: Engine noise and oil pressure correlation, engine oil pressure, chevy astro

Question
My 1998 Chevy Astro 4.3 118k miles has been making a sound that normally occurs in the drive position, but at a stop, and sounds exactly like a loose muffler/exhaust pipe. After several inspections, the exhaust system is in good shape.  However, the other day I was in a drive through (in gear, brakes applied) when I heard the sound again, but this time I noticed the engine oil pressure needle bouncing in rhythm with the noise. Putting it in neutral stopped the noise, and needle bounce.  This occurs several times a day while driving.

Is the oil pump in need of replacing?
Thanks.

Answer
Hello Paul,
Can't say that I know what the noise is, but it sounds similar to a truck I had that the thrust bearing on the crank was bad, and the flex plate would barely touch the rear of the block at times.
Get under there and pry the crankshaft to the front, then rear, and see how much travel it has. Shouldn't be very much.
I was thinking cracked flex plate when I found that one, so you might check that also.
A rod bearing with excess clearance could allow the oil to escape once per revolution, and possibly knock at real low rpm, but be better oiled as the engine speeds up without the load on it, and quit knocking.

Van