What could cause a rattling sound inside one of these feeders. I've had the lid off, the interior is empty. There is a small black hole at the inside base of the feeder....is there a spring on the other side (pipe side) of this? The rattling is not constant, nor is it associated with filter pressure, or time of day, temperature that I can see. I don't use it and am thinking the best way is to cut it out of the system altogether, but thought I'd ask here first in case someone can shed light on the issue. please see pic.