There is a small pipe above the fill that was probably where a button whistle was, if it ever existed.

I wouldn’t be surprised if the whole thing is improperly installed. In other words, the button whistle may be the only vent and there may be no associated main vent with weighted cap. Cases like this are very dangerous because the tank can’t vent fast enough while being filled and can lead to tank failure.

Not saying that’s the case here, but the amount of leaking can also be indicative of back pressure from inadequate venting.