Back in... man... probably 1996 I went to work with a thermal tumbler filled with some hot beverage or another. I settled in and took a swig; the liquid was good and hot and I kind of marveled at how well the tumbler worked and how it was nice to have a warm drink in my cool environment and after a chilly ride in to work. As it sat, sealed, on my workbench, it started to make a noise; a high pitched squeal. I was intrigued by that., and the discussion that ensued after pointing it out to a co-worker and my boss has always been vexing. I see both sides of this argument. My take was that like any sealed vessel with a higher internal temperature, the pressure in the vessel would be higher than the cooler outside pressure, thus the tumbler was effectively venting the pressure which was greater than the seals could, well, seal. The differential was greater due to the cool work environment than it would have been inside my home which was quite a bit warmer. In short, I thought gasses were escaping due to being forced out as the pressure inside the tumbler was higher than the external pressure. My boss disagreed. His position was that the noise was created by air rushing into the tumbler. His theory was that the liquid was cooling, and so the pressure was dropping at which point regardless of the cooler air outside, it would rush into seals at a matter of a vacuum state developing. In retrospect I'd say both instances are true based on pressure and temperature changes over time. I suspect that initially the tumbler was purging excess pressure, but at some point this is reversed and the tumbler then starts to draw in atmosphere. I think this is observable when putting a bowl filled with water and covered in plastic wrap in a microwave. Initially the plastic remains flat with room temperature water. Once the water is heated, the wrap expands with the increasing pressure due to the liquid heating enough to create water vapor which then expands. But, as the heating cycle ends, the plastic wrap then shrinks downward and meets the surface of the water, effectively vacuum sealing itself. What are your thoughts on this?