Thanks . that is a plausible answer. I had'n thought about a manifold leak. It was not making that noise as I drove it home so I was suprised this morning. Mostly I wanted to feel safe to drive it to the mechanics shop about 3 miles tomorrow. Run by a bunch of ex ford dealer mechanics and Service Manager. I love this truck it has never given me any major problems and at 144,000 miles I want to drive it 100,000 more!