Hi John
When you edit the audio clip, try the following steps.
Click once anywhere on the waveform. It doesn't matter where. This step just ensures the playhead is positioned somewhere.
Tap the End key on your keyboard to position the playhead at the end of the clip.
Press the Left Arrow key twice to move the playhead just a bit nearer the beginning of the clip.
Now insert your silence at this spot.
What usually happens is that Captivate is being helpful. It doesn't understand you actually WANT the silence there. So it dutifully trims off the silent bit. By positioning the playhead as I instructed above, you actually insert the silence between the ambient noise. This normally causes Captivate to respect the added silence.
Hope this helps a smidge... Rick 