Thanks to ILUVMILS, I was put on the correct track. It is indeed the Air Injection pump. I put the car on ramps, did a cold start and was able to pinpoint where the sound was coming from. So now (knowing the price of a new pump) is there anything serviceable, or do I need to get a whole rebuilt unit. It sounds like the bearings are going out. Should I wait till it fails catastrophically and throws a code, or should I be proactive?

TIA.