content: Yep, it's the Gmail tab, but it works like this: I can only hear it when I'm on the gmail tab, any other tab is silent, in other words, there is no sound icon.  At first I thought it was some sort of static that was "filtering" at some point, but the sound is indeed different than the regular static sound I get from certain sources.  On the other hand, thank you for the tracking protection, I though I had changed it long ago... it wasn't the case. I must admit, it could be an issue with the receiver/tv set since both are pretty old... but then the sound is too distinctive. Maybe something with an open audio channel in the gmail tab for the audio notifications for new messages. Oh well.