Apple Final Cut Express HD - Manual (Page 619)

Chapter 44 Mixing Audio in the Timeline and Viewer 619
VIII
5 Click the Level Keyframe button to mark four keyframes in a row.
The two inner keyframes surround the problem samples, while the two outer
keyframes are placed a few hundredths of a frame outside of these.
6 Drag the part of the level overlay between the two inner keyframes down until the box
indicates –60 dB.
The unwanted noise should be gone, and the rest of your clip’s audio is not affected.
The problem samples
The overlay looks
something like this.