Either under or over torqued can do it. When tank gets old the plastic threads can fray to make for added torque needed to pull up the gasket tight. On both mine they hiss until I ever so slightly torque more than I can by hand with a big pair of channel locks to make it seal and stop. I'm talking a very small amount, easy to overdo it and strip threads.
Both mine been like that forever.
The cap MUST seal or coolant boils..............