Sounds like a head gasket. The Neons had that problem.Usualy the water goes out the tail pipe, thats why you don't see it in the oil.I would hear mine gurgle after I shut it off, the pressure in the cylinder was pressurizing the system. Usually the gasket will break between one of the cylinders.Have someone do a leak down test to confirm it though. The only thing that makes it hard is everything is so compact and tight.If that is the problem it will get worse, and then bam all of the water will go right out of the tail pipe, and you will be sitting on the side of the road. The timing belt and water pump needs changing every 105,000, so if your close to that, it would be better to do it all. There are other things to look at, but how many miles and what kind of oil do you use? I have 450,000 on my Neon, so I kinda know whats up with them.