Help diagnose engine noise

1970 351W, This started not long after I did an oil change using 15w40 diesel because it has zddp and my usual oil wasn’t available. It only happens when under load, as you can hear it stops when I let off the throttle, can’t get it to replicate just revving in the garage, there it sounds great so very difficult to pinpoint. Hoping someone with an experienced ear might be able to help.

I’m no oil scientist, but I feel 15w40 for a diesel isn’t that good to run in these engines. For quite a while I was using 15w40 in my 68 Monterey and intermittently there would be lifter noise. I started running 10w-30 break-in oil. I found the Comp cams oil at Summit was reasonably priced by the case. The engine noises cleared up after some driving.
I cannot hear the noise in the video with the computer I am using.

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I’m in Canada, usually use Lucas 10w40.

I can’t tell what that knocking is. Hard to tell what it is. Almost sounds like something is loose. I always use the Valvoline VR1 racing oil for the extra zinc content, never had an issue getting it. I hope you get the noise figured out and let us know


Id start by changing the filter . I always use K&N white oil filters . Less pres drop and the 1in hex on end is handy . With the cost of engine repair ,a $16.00 filter is not out of line.

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Hard to tell with computer audio, but if it’s coming from deep within the engine, and happens under load, it is likely a bad rod bearing,

here is the next step…remove the oil filter, cut it open. Poke a hole and use sheet metal snips-----do not saw or use a grinding wheel as that will create debris and distort the inspection. Look for metallic, if so, what kind of metallic. Is it sludgy? Sludginess could be due to the diesel motor oil. Inspect and let us know.


I would start by changing the oil (and filter) type back to what you were previously using.
See if you can eliminate that as a source of the issue.

Take the advice on cutting open the old filter, to see if you have signs of internal damage.

I second using Valvoline VR1. If you can’t find it, you can usually get STP zinc additive in a little half-quart jug to add to whatever oil you were using before.

Let us know what you find!


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I hear a ticking so I’m going to say a lifter. Change back to your usual oil flavor and test.

I had a “tik” at idle once, a loud tap under load,
mechanic had a listen and was adamant it was a pending doom from a BE bearing not long for this world.
I pulled the engine, to have the exhaust manifold fall in half wen i removed it from the heads….!
I went ahead and reconditioned the motor any way.
The engine builder said there was nothing terribly wrong with the engine just old age.