Went for a drive a few days ago when the weather was sunny and in the mid 70’s as probably the last convertible day of the year in the Houston area. At about 50 miles or so the engine started making a terrible knocking noise, but no smoke, power loss, etc…. Stopped at an O’reilly’s to check it out, at idle the noise was quieter but still there and increased with RPM. Put in a quart of Lucas Oil treatment at the recommendation of a mechanic who happened to be there as he thought it might be a lifter problem and it was a half quart low limped it back home and noise still there. Pulled the intake and valve covers and all rocker arms are tight, push rods are straight, and lifters appear to be fine. Still starts and idles fine so not sure where to go from here? It’s a 1977 403 with big block heads on it that was built around 1982 and has been running strong, although I don’t know much about the history of it. Could a piston rod bearing be bad causing that noise?

 

Check for a cracked flexplate and loose bolts there as well. What is your oil pressure? Any recent drops in pressure?

 

Oil pressure stays the same-here’s a video with sound that might help. Flexplate makes sense though?

 

Exhaust leak???

Don W

 

That sounds like it's coming from the rotating assy to me.

 

Agree, sounds like a loose wrist pin.

 

Stop...that sure sounds like rod knock.
Before you commit to a tear-down, confirm with an inspection of the filter contents for flakes/shavings, and see if the drained oil looks like silver paint.

 

How are you guys getting the video to play ?

 

Seems to me a lot of noise to instantly pop up? Nothing started out slow and got louder? Sounds like a flexplate issue or bolts there... Time to start taking things apart...

 

I finally got the video to play. Sounds like valve train to me. Too fast for bottom end ? First thing i would do before running it any more would be to take the oil filter off and cut it open and see if there is any metal in it. That might help decode which way to proceed.

 

Pull one spark plug wire at a time and see if the noise changes tone.

 

Lifters, rocker arms, and push rods were all in great shape, turns out it was a problem in the rod bearings and one of them apparently spun…. Now it won’t run and the oil is a shimmering copper color! Time to rebuild the engine I guess…more to come once I start breaking it down.