Hello All
Turns out it was not the exhaust manifold and was not the fan clutch (I did, however, fix the sheared bolt on the manifold).
The cause of the noise was a component of the emissions control system whose function I have yet to determine. There are two approx 1" pipes that come from the cat converter (one from the front, one from the back) wrap around the side of the engine and then disappear into the difficult-to-access passenger-side front of the engine. For most of their run they are metal, but for an approx 8" stretch as they transition from under the car into the space occupied by the engine (right next to the exhaust manifold!) they transition to hose. One of those hoses blew out and that was where the noise was coming from. I replaced the hose, but first trip out when I went up a hill the power started to drop, the hose burst again, and the noise returned.
The purpose of the pipes is not described in either book I have on the car. It is *not* one of those systems that injects air back into the exhaust manifold at each exhaust port. I will open another thread to ask for help identifying.
Best,
Jim
