I use Audacity, too. My hint is to zoom into the source sound until you can see a pattern in engine strokes or whatever pulses they are. You then make a selection a little longer than the desired resulting sound and press 'Z' for finding nearest zero of both ends of the selection. I prefer to start my selection below zero line when the beat curve is rising and end it above zero with the curve rising again, trying to catch a rhythm pattern and get the same uphill curve into both ends.
One more note: Don't make the sound clip too long, or it may make strange things. I noticed that with a 30 seconds loop offline sounded good, but online the volume decreased dramatically.
