First, the only way to check the pressure in an expansion tank that measures the static air pressure, is to relieve all water pressure from the other side first; otherwise, it will just read the water pressure. The static pressure should be about the same as the boiler's water pressure. That way, the diaphram doesn't get stretched too far and it will last longer. If you didn't get any water out when you checked the pressure, the bladder is intact - it might not have the proper pressure in it, but it is intact.
If the pump is not moving the water properly, it can stay IN the boiler too long and intermittently flash to steam, which will makes some sounds. What you are hearing, is hard to tell without being there, but it could be bubbles of steam collapsing.