Most of the water connections in the Highflow 400 are visible from the front with the main cover removed which simply lifts up and out. Use a torch to look closely at the joints. If there has been a leak there is often evidence of it on the next surface beneath the joint where the water has dripped. If you can hear it I suspect that it is external.
If you still can't see it, it could be the main heat exchanger or the auto air vent. These are located inside the combustion chamber. If you don't know what you are doing I would call someone in at this point.
Kevin
October 2005