Although the heating element (gas or electric) is turned off when the BULK water temperature reaches 140 degrees, the gas or electric actually causes the water in contact with the heating part to boil. The steam bubbles move away from the heating section and then "collapse" as they give up some heat to the surrounding water. The collapse of the bubbles is what you hear.
I would find it hard to imagine you can hear this noise inside the TT (unless you're REALLY being quiet) as 5 sides of the heater are insulated with foam, which also provides sound insulation. I'm assuming you're only hearing this outside.
1988 Coleman Sequoia - popup (1987-2009) - outlasted 3 Dodge Grand Caravans!
2012 Roo19 - hybrid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado 1500 Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10