Creating a sound that loops nicely

[Raw Kuts]

I have some sounds (engines running) that I want to be able to loop and have it sound continuous. The sounds are already basically constant, so I don't think it should be too hard, but I don't know where to start.

If they were pictures, I would know what to do in photoshop to blend the edges together, but I've never worked with audio before. I've been messing around in audacity for a while, but I haven't gotten anything that loops nicely yet.

Any tutorials/tips on doing something like this?

edit:
doing something like this is as far as i've gotten, if you play that it's not too bad. I just copied and pasted the track and at the beginning and end I made an envelope to hopefully make the sum come out to be constant energy. There's still a very audible transition though.

[Great Tiger]

Sound Forge ftw

[redrum781]

upload what you are working with and and what you have so far somewhere. where did you get this sound?

[Raw Kuts]

I downloaded the sound from one of the countless 'free sfx' repositories from google so it's not the best quality to begin with, but it's one of the few that I found that has an engine sound that is just steady and not all dopplerified.

This is what the original sound was: http://www.rawkuts.com/game/Jet%20En...20constant.wav

This is what I have right now: http://www.rawkuts.com/game/hovercra...ine_loop01.wav

These are the audacity projects I've been working with: http://www.rawkuts.com/game/jet%20en...20audacity.zip