That’s the speaker being overdriven to the extent that it’s voice coil is hitting the speaker magnet. The little on-board speaker isn’t meant to reproduce the sound very accurately, it’s just meant as somewhat of a monitor to let you know the Macchiato is making sound at all. So it can be misleading when it causes problems like this. In fact, the next version of the Macchiato (which we are currently working on) will not have an on-baord speaker, partially to keep issues like this from happening.
To cure this problem, just lower the volume of the Macchiato.
Sorry about this issue.