problems for Audacity 2.0.2 for a Macbook OS 10.6.8

I used Audacity with my old OS without a problem. I have now upgraded to OS 10.6. Now when I use Audacity I get a high pitched squeal when I push the record button even when I do not have any external devices attached. Can anyone help?

If you have Built-In Microphone, Built-In Output (Speakers) and Audacity > Preferences > Recording > Playthrough all selected at once, that’s a feedback path when you either press Record or enter Monitor Mode.

Wear headphones or earbuds.