Had loads of time waiting for some pax to turn up for a spin in a Bonanza. When the opportunity presents itself like this, I like to do an ultra-thorough pre-flight. I pulled the prop through several times. Heard a whistle on one of the cylinders. Opened the cowl and had a look. Small crack in one of the cylinder heads. It probably would have continued to run through to the next 100 hourly, but it might not have done so.