I had an engine noise that I was afraid was an exhaust leak, or worse, a bad rod bearing. I think (I hope) that I solved what was a minor problem.

There are two metal cooling lines running from the radiator to the automatic transmission. They are supported by a clamp bolted to the underside of the car. I found that the lines were rattling inside the clamp. I shimmed around the lines and reinstalled the clamp. No more rattle and I'm fairly sure the noise is gone. Note that I used a cloth layer from an old lawnmower drive belt hanging around in my garag. I needed a thin shim and i assumed this material would be heat resistant.