93 Civic Coupe EX, 1.6L, 5sp, 250k miles

I've had a slow water leak for several months and I can smell antifreeze on hotter days. From time to time there would be a hissing sound (stream) from behind the water pump area. The engine has lots of miles (over 150k) and I have no service history.

Hoping it would be something cheap I replaced the connecting tube o-rings and the hose which connects to the tube.
Took it for a quick drive with only water in the radiator, same sound from behind the water pump.

Could it be the water pump?

 

The noise might be caused by air trapped in the cooling system. Push the heater lever to max heat, drain all the water (=bad) out of the system, refill with 50/50 coolant, and then bleed the system: