Mine did something very similar about a week after buying it, there is a plug in the head from the water jacket, it was put there whilst casting the head, and they leak, very slightly, meaning around the spark plugs fills with coolant, take the ht leads out, and with a long straw suck the water out from around them, but do not remove the spark plugs until the water is gone. I then replaced my spark plugs and ht leads for good measure.
Hope this helps