Last night while sleeping on the portside setee I was awoken twice by a noise that went: Chunka-chunka-chunka-chunka-chunka (I'm paraphrasing a bit). It sounded like it was coming from the bilge (but may have been the Par pump since I was laying on top of it). I checked both and they are both functioning fine. Cannot hear the noise under normal conditions or when the 12-v fan is on. Any ideas?