What could be causing the bizarre whistling sound that is coming from the pipes around the en suite and main bath?
All drains must be vented to the atmosphere, to allow water to exit smoothly..  Vent pipes (on the roof, can act as pitch pipes, as the wind blows across them.  

Clogs (or restrictions-soon-to-be clogs) in the drain pipes can cause water to rise into the vent pipe, changing the pitch as the water level varies.

Sometimes a bird can block the pipe with a nest or other critter can bodily block the pipe.

In any case, checking the main vent pipe is a the first order.