I'm hearing a faint knocking sound coming from one of my returns. The same return also has a few bubbles coming out of it. They don't come from the opening of the return, but they come from the small space between the plaster and the return fitting. Could this be from my SWCG? The flow from the return is no different than the other four returns of the pool so nothing appears to be obstructing the flow from that return. I've noticed this for months. Any ideas?