I bought this 1998 Sea Ray 290 Sundancer recently and it used the have a very clear vacuum sound whenever the toilet was flushed. A week ago the vacuum sound did not happen, only the sound of the water pump putting water into the bowl. So instead of using the toilet, I have test flushed it since and about every 8+ flushes the vacuum sound happens, while every other time the water simply fills the bowl more. Before I pay hundreds to have the system serviced, any ideas to what I should do? The holding tank is empty. All ideas are welcome!