WAV to MP3 CONVERSION ADDING SIX SECONDS AT END

When I used AUDACITY for converting wav files to 192 MP3s before, I did not experience this problem: I narrate audio books and must have precisely 3.5 seconds of room tone at the end of each chapter. This is fine in the wav file, but once I convert it in Audacity and export it as teh 192 mp3, it shows up with six seconds added - even though the source has 3.5 at the end.
HELP!!! Does anyone have any idea why this is happening?!! Many thanks for any suggestions — JAMIE

How do you get the room tone? Just stop talking for five seconds and cut it down? What happens to the length of the performance if you cut it at exactly your last word – just as a test.

Koz