SBC circulation pump impeller failure.
 

Yesterday running at speed I heard a rattle (music was loud so took a minute to pick it up. Throttled down and rattle was coming from circulation pump and engine quickly got hot. My theory is at speed the raw water forced it thru like a cross over. Anyhow, took it part and the impeller failed at the hub/ shaft. Pump is about 10 years old and has about 1000hrs on it but what a strange failure . Anyone ever hear of this? I at first thought a piece of FE got loose but I see no marring on the bronze impeller and seems like that would leave a mark. Tried to upload a pic but a free member these days and I guess I can't.

Yes, I have see that not often but more than once. Both small and big blocks. You are lucky you heard the noise to give you an idea where to look otherwise it can take a while to figure out the problem. Especially the first time you run into it.