Weird whistle

[icon787]

Hello, over the past few weeks (audio file attached) i have noticed a whistling sound coming from one of the bathrooms and it appears to be coming from the wall either behind the sink or toilet (not entirely sure though). Based on my observation it starts when someone opens a faucet or flushes a toilet on the same level (2nd floor). It either goes away by itself or when i open the faucet in the problem bathroom. Any ideas? Will appreciate any input before i call the plumber.
Thanks

 

[Twowaxhack]

Audio file not playing for me.

Most of the time whistles come from the toilet ballcock, aka toilet fill valve.