The fact that you're getting it on different tunes suggests a mechanical problem like a leak to me too. But I don't have any suggestions about how to find it. I wonder if there'd be any "blowby" that might create a "soot mark" if there's a leak?
Sometimes you can hear a leak "whistle", but you might need a stethoscope to be able to isolate that from other engine noises.
- Jack
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