Does it happen when you’re not using the pump and just hooked up to an outside water source? There should be a water line under your sink cabinet that runs to your toilet, it’s hidden under some trim that might be a good place to look also in the bathroom closet under neath the closet floor where your propane tank should be behind that should be a place to access a water line. Have you turned it on and crawled underneath to see if you can find it?