Meltedpanda,

Thank you for your answer.
I will have to trace the mentioned venting lines. My SL has a charcoal filter.
The filler cap is hard to remove, and have to check that one somehow.
I siphoned 3 liters of gas into a tall clear flower vase, and no sign of water.
Reading further the many answers on-line, one was interesting. The strainer in the tank is fully submerged when the tank is between full and ¼. That may explain the trouble when the tank is less than ½ and parked on a slight slope and the Internal Tank Filter (strainer) is clogged (which I suspect). This does not explain that gurgling sound.