Yes check your drain plug. If the bubbles of air are in the bottom of the bowl that is where the problem is. Air bubble from a drain leak will be large. Air bumbles that are very fine and high in the bowl are a fuel line, which is rare unless something let loose. Go to where man last touched that is where the problem lies.Travis,if the drain is one of the ones that is glued in and not threaded into the bottom of the bowl,that is your air leak.