OK, guys- - - -I've been listening to engine noises for almost 60 years. Not being there leaning over the fender makes this one a little challenging, but my opinion hasn't changed. Here's why:
The hard-sounding BUMP-BUMP-BUMP as the engine is accelerating, then leveling off and almost going away as the load goes away is classic main bearing symptoms. The crankshaft is bouncing up and down on the mains (particularly the rear with the inertia of the flywheel and clutch) like a sledgehammer. A cracked piston skirt causes a somewhat similar noise, but not as heavy or as distinct as a main bearing. This one's definitely a main. Pulling spark plug wires one at a time can isolate a bad piston- - - -it won't change a main bearing noise.
Rod knocks happen on a "float" as the engine accelerates and the speed levels off- - - -the loose bearing hammers up and down on the crankshaft journal because of excessive clearance and/or crankshaft wear. Rods make a light to moderate RATTLE-RATTLE-RATTLE noise, much faster than a main bearing knock. If several rods are involved, the frequency of the noise is quite rapid.
Valve lifter and rocker arm noises happen at a lower frequency than connecting rod or main bearing noises- - - - -the camshaft turns at half crankshaft speed.
Now- - - - -pull that engine and tear it down while it's still rebuildable, or carry a big basket around to haul the parts home when it starts chucking bits and pieces out the side of the block!
Jerry
