I changed over to using a Mac as my primary machine (again) around 12 months ago. Its a great workhorse for a laptop, the SSD and 16GB of RAM make it perfect for AutoLab. However, every now and then I notice an annoying sound that I couldn't work out where it was coming from. Admittedly I didn't try too hard to find the source, but it wore me down today.
The culprit? iMessage.
Every time someone signed in or out of Google Chat, Jabber etc, I got an annoying swoosh sound. When I found it, I certainly felt like a noob.
Simple fix, head into iMessage -> Preferences -> Alerts. Change these two events to have no sound: Buddy Becomes Available and Buddy Becomes Unavailable.