I drove my Volvo all over this morning. When I got home, I changed the oil. After the oil and filter were changed, I started the engine, let it idle for a bit, shut it down, and confirmed that the oil level was correct. It was. I then changed the intake manifold gasket and tightened the nuts on the downpipe because of a slight leak. While I was under there, I noticed that the hot air pipe on the exhaust manifold for the air filter was rusted so I pulled it off. When I put the car back on the ground and started it up, it has a very distinct knock that increases with engine RPM. I can feel it when I put my hand on the valve cover. What could I have done wrong? It sounded fine before I did the gasket. The oil light comes on with the key in "on" but shuts off after the engine starts. Is there a sensor I could have accidentally knocked loose that would cause a knock?