So here's the deal, maybe this will help someone else with the same problem. A while back I wrote a post asking for help with a problem of air getting back in my cooling system, anytime the rover was off camber the slightest bit air would rush to the top of the heater core.. I finally figured out why it kept having the "waterfall" sound behind the dash. The coolant level was too low. it did not look low and was always at the correct level. (never overheated) The fix for me... I started the rover from a cold start with the overflow bottle in place and the cap off. I topped it off up to the neck, as the coolant crept up to the top I would open and hold the throttle in place and the coolant would be sucked down by the engine. I did this until 4k rpm, The whole process took less than 5 min. (My rover did not get to operating temp during this time) with the rpm's at 4k I used one hand to hold the throttle open and one to put the overflow cap back on. Now I have no issues at all.
cheers.
cheers.