boiling/whistle noise for about 10 seconds after shutdown

Hey guys I just got my car back from the shop, my heat exchange was replaced and mutiple flushes were performed. I only owned my car for about 60 days driven it about 3 weeks of them 60 days cuz it was in the shop other days. Anyways I noticed i had puddles/drips of fluid, no smell or taste/far as i can tell no color....i would think its condensation from the AC but i wasnt running the ac. Well i ended up getting a gas leak the other day brought it in got it back today. The puddles were still occuring? Also I had Ferrari do a flush because my coolant still had oil in it from the heat exchanger fail. I showed it to some buddies that flew into town and i revved it up a few times after i warmed it up drove em around the block....noticed the puddles/drips as i pulled it in the garage, they are coming from the center of the car not sure if its front or rear. Then i shut it off and i heared a boiling/whistling noise for like 10 to 20 seconds... what could that noise be? Coolant temps are normal!!!