Well, mystery solved! And potentially a little embarassing since I should have noticed it straight away. Turns out it is not an exhaust leak at all, actually more of an intake leak! The throttle body and rubber hose had pulled off the intake port on the head. Somehow. I don't know how that would happen, the clamps seemed tight. It hasn't been dropped since I've had it, but I haven't had it long. But the whole T.B. contraption had slid back a bit further into the airbox. So on compression the air in the head was forced into the throttle body where it 'ricocheted' back toward the head. That's where I was feeling the 'exhaust leak'. So, false alarm. All is well. Thanks for your input. Now...back to your regularly scheduled program-