MacBook Pro "clicking" while muted when showing a message dialog in Windows on Parallels - what's happening? I am experiencing odd "click" noises from my MBP, while sound is muted, it's unplugged from the power source (ie running only on battery), and Windows in Parallels shows a message dialog.

To reproduce:


Unplug the laptop (right now, 94% battery)
Ensure the volume is muted
In Windows on Parallels, do something that shows a message dialog
A loud "click" will be heard. It sounds plasticky or staticky, like the pop you get when plugging in a speaker to a powered amp, and I can't trace it to either speaker - it seems to come more from the back center or back right (maybe the right-side speaker) of the laptop. Two seconds later, a second, slightly quieter click will occur while the message dialog is still open.


The laptop is muted, absolutely.

Is there another sound source in a MBP, apart from the speakers? Why would this happen when showing a dialog - could it be a mechanical change in the laptop itself, caused by something to do with the video card?


Early 2011 model MBP
Yosemite, fully up to date
At the time, running Parallels 9 hosting Windows 7
Volume does not have to be muted for this to happen, but it does happen when the laptop volume is muted, making me think it may not be a sound from the speaker (?)


Edit: The original question was that this happened for a low battery warning. Since it's now happening with a nearly full battery, and it seems to be related to message dialogs, the fact the messages were about a low battery is a coincidence.
The pops you hear are due to the sound card turning on then turning off.  It is more obvious if you connect your laptop to an external sound system.  Unfortunately it isn't something you can control/change on your system.
I'm guessing that Apple does this so that the sound card doesn't play interference sounds (hums, whines, etc) from power sources, cell phones, etc.

Although it might be more obvious with the Windows dialog and alert sound, it is not just Parallels.  I have the same problem in Parallels, VMware Fusion, in every Mac application, and even the Finder.

For example, try pressing the volume up and down buttons on your keyboard and listen very carefully.  Just before the volume change sound plays, you will hear the pop as it turns on.  About 3 seconds later, you will hear another pop as it turns off.  (If the volume sound doesn't play when you press the volume buttons, hold down shift).

Another example is to play a movie in Quicktime Player where it is completely silent at the start.  When you press Play, you will get the pop.  The "off" pop will happen about 3 seconds after you close the movie window or quit Quicktime Player.

If it bothers you a lot, a total hack workaround I've found is to get Audio Hijack Pro.  Install and run it then from the Audio Hijack Pro menu, select Install Extras and install the Instant On option.  In the main window, select the System Audio entry and click the Hijack button.  You will hear the pop as AHP turns on the sound card.  Since the sound card is already on, other apps like Parallels won't cause the pops to happen. Simply hide or minimize the AHP application and it will keep the sound card active.

AHP is free but any recordings over 10 minutes will have noise added.  Since we are just hijacking and not actually recording anything, you shouldn't have any problems with the free version.