Hello. I have a 2001 wrangler with the four cylinder, everything is pretty much stock. I was driving the other day and out of no where I encountered a noise that sounded like an exhaust leak. I pulled over and narrowed it down to a factory hole in the exhaust manifold. It is at the bottom of the manifold just before it connects to the pipe leading to the muffler, but on the manifold side. The hole is threaded. It looks like a sensor could go there, but there is no evidence of any wires dangling or anything being disconnected. I went to a few auto part stores but that proved to be worthless. Does anyone know why this hole is there and should it just be plugged with a bolt? If it can/should be plugged, does anyone know what size?