Editing : Editing Basics : Audio Clips and Waveforms : Avoiding Clicks and Pops

Avoiding Clicks and Pops
If an edited clip begins or ends at a point of high amplitude, you may hear a click when Pro Tools plays from one clip to another. In order to avoid clicks or pops do any of the following:
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Make sure that the start and end points of your selection are as close as possible to the point where the amplitude of the waveform tapers down to meet the zero-crossing line (the center line of the track’s waveform display). If necessary, use the zooming tools in the Edit window (see Zooming Options) to display waveforms in greater detail.
 
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With Pro Tools HD and Pro Tools with Complete Production Toolkit, use the AutoFade feature to apply real-time fade-ins/outs to all clip boundaries that do not touch or overlap other clips. See Using AutoFades for details.