Sounds a LOT like my 3.0L before the HG went. The motor ran fine...but, every few months I'd notice the coolant was low. Didn't see any leaks and couldn't figure out where the stuff was going.....and never paid much attention to the problem.

Then one day, I'm driving into the mountains to do a little hiking and started to hear that water rushing sound you mentioned...then noticed my temp gauge kinda of being all over the place. I pulled over and found out the coolant was low (Don't ask how I checked it). So I filled that up and drove off. About a week later my head gasket officially blew.....in a VERY large and thick cloud of white smoke.....I filled a fair size parking lot with smoke.....Impressive for such a small motor.