That is actually going to be my next tutorial on the sound page but to answer your question in a nutshell:
1. import files into audacity
2. cut up the sample (anything over half a second is good for motor noise)
3. make another copy of the audio track and reverse the second copy
4. speed up, slow down, change pitch and do whatever operations are necessary.
when plotting the graph into train editor, y=100 means the default pitch will be played (the noise will be played/looped as is, when y values are higher than 100 then pitch will be raised while a low y value would lower the pitch.)