This may be an old trick to most out there but if you haven't heard of this before here is my trick to ruling out an exhast leak. I have an old stethoscope but instead of the solid metal rod that normally is on the end of an automotive stethoscope, I put a pushrod into the end of the rubber tube. Because the pushrod is hollow, if you run it around the edges of the exhaust gasket and there is even a tiny leak, it sounds like a super high powered leaf blower or air from an air hose blowing in your ear. If it is a tap from the valve train or any other mechanical knock it simply sounds like a ticking noise like you would hear with a normal automotive stethoscope. No it doesn't blow enough air through the hollow pushrod and rubber tubing to damage your ear drum or anything, but if it is moving air you will definately know the difference and it is fairly easy to get the end of the pushrod down into tight areas to find those hard to reach exhaust leaks. Ok, well that's my two cents.

 

Was wondering if you ever tracked down the noise?