Update: My exhaust leak is located at the exhaust pipe upper flange on the front of the engine. As the engine heats up, the sound is reduced, but on a cold start, it's very evident.
I applied a bead of JB Weld Extreme Heat on the flange above the heat shield that protects the oxygen sensor., being careful not to stake the flange nuts. After air curing overnight, the sound was gone on start-up. I've driven the car for a few days and all is good. Even if the bond joint eventually fails, at least I know the source of the problem for subsequent replacement of the exhaust pipe/catalytic converter, which runs about $350. If it lasts indefinitely, I got off easy, spending $8.
A shop quoted me 14 hours of labor for the replacement which I thought was a little excessive, so I obtained the replacement procedure and it's straightforward enough. Remove the oxygen sensor, loosen the flange bolts, go under the car and loosen the next section of pipe, then remove and replace the offending component, replacing the exhaust gaskets. I can't see where that would take more than two or three hours unless a nut is stuck or a stud breaks.
If the bond joint yields, I'll replace the pipe and provide an update.
This occurred just under 80k miles.
CCC