This past weekend I went into some shallow water and one of my engines sucked in what sounded like a rock. It rattled around a bit before I got it shut down. Now that it's clear and out of the water, that engine makes a rattle sound that wasn't there before the incident. It's not very loud but now both engines sound very different. Before putting the boat back on the trailer I noticed that both engines seem to be performing correctly. There is no visible damage to impeller but I have not taken anything apart. Any guess what might be making the rattle?