I have a relatively new toilet (2 Years Old), which sometimes double flushes automatically, and other times the water level in the bowl drops a bit, when it is not used for a couple hours. When we flush it, it bubbles, as if air was trapped. I have one other toilet, which is in the basement (ejector pump type), which uses the same outlet to the Septic Tank. They have separate roof vents. What causes this double flushing and water level drop ? it is a Crane made toilet.