[Mister Moo]

A bad seal between gasket/basket rim usually involves pretty obvious downward dripping of coffee or coffee-spew you can easily trace as it dries on the grouphead or pf. Are you able to witness a steam, water or coffee leak when you hear the sound? Is the sound behind the housing or coming from the grouphead?

Since you now live in the exotic world of big boilers, heat-exchange systems and siphoning E61s you're a bit out of my range. For all I know there is a periodic whistler built into each and every Brasilia specifically to indicate everything is working spot on. But I doubt it. You want to track that down when it happenes next - something is almost certainly leaking.