Around 97,000 miles I kept getting a "engine coolant low" light come on. At first I thought it was not a big deal and just added some coolant to my reservoir. After a 2 weeks the light came on again, and I checked and my reservoir was empty again. So I added a little more, and sure enough 2 weeks later my car had gone through about a gallon of coolant, but with no visible leaks. I finally checked my oil and found a nice mixture of coolant and oil. I did some research and found this was a common problem because GM used crappy gaskets which erode over time causing coolant and oil to mix. To fix requires the upper and lower intake manifold gaskets to be replaced.