On small aircraft engines, I have a small exhaust leak sound like everything from a tappet to a serious rod knock. More concerning at 9,000 feet. Just a thought.
SWMBO's '66 Coupe 289, 2-V, C-4(Pony Interior, Rally Pac, A/C & Tiffany Stripe) & An absolutely rust free '65 289-2V C-4 coupe project. Now almost done. Only took 13+ years.