Soundtrack Pro 2 is the primary piece of software I use to edit audio. In producing work for a client, I recently discovered an annoying bug when sweetening sound files that my client had recorded on only one channel.
What I discovered is that Soundtrack Pro generates a loud screech (feedback) when I try to preview the Noise Reduction filter using the HUD popup window. I also encounter feedback when I audition the two limiter filters that appear under the Effects menu (Process>Effects>Dynamics).
Fortunately, I found an easy-to-use workaround. I simply convert the file from mono to stereo and apply the filter or effect that I want. Afterward, I convert back to mono.