My car is a toyota land cruiser 2007 with over 179,000 mileage manual transmission. A few days ago while I was driving, the alternator light came on the dashboard, I pulled over and checked the bonnet and saw that the alternator belts had cut. I got an auto mechanic to fix the alternator belts and the light on the dashboard went off and the car was working normally. By the time I drove it home, which was about 15 minutes later, I heard noises under the bonnet like hot water boiling. About 3 hours later, I checked the radiator and observed that there was almost no water and I put in a lot of water into the radiator. By the time I drove it the following day, the radiator was almost empty. My observation is anytime the car is cool and stationary when I fill the radiator there is no leak, but when I drive it for about 30 minutes or more and I park the car and check the radiator after the car has cooled down, the water is gone. While I am driving, the temperature gauge is fine, the engine does not overheat, sometimes I hear the hot water sound when I get to my destination, sometimes I don't, actually most times I don't. I don't see any pool of water underneath the car when I park. My auto mechanic checked, he couldn't find what was wrong on a visible inspection. One thing I just observed today is that the radiator cap used to be really tight, but since the incident its not as tight, it's kind of freer to open and close,but I am not sure if this could have anything to do with it, but the water in the radiator is still disappearing rapidly and I have to fill it more than once a day.  Can you assist and point me in the right direction.