Have a Fighter's Garage AST with maybe 5,000 miles since install. Sometimes when the car is cooling down, you can hear a whistle coming from the AST cap. Running a 16# cap on the regular filler. Haven't noticed any coolant leaking, but if air is escaping, coolant might be able too as well. Anyone else have this issue?

 

At cool down the coolant system fluid is contracting and is sucking in coolant from the overflow tank along with anything else it can suck in from leaks in the system - such as air.

What you are hearing is probably not Air escaping. It is probably sucking air into the system (which is not good).