I find it really hard to make short sounds longer in length, I don't know if this was a good idea but I was trying to take a bike by and make sound like a bike engine that's constantly running. Also I was wondering what techniques one could use when editing sounds that are sort of like the Reedman scene in Transformers ROTF where they used a "buzzing magnet" that makes a short little buzz, and make it sound like a continuous thing while it transforms (Sorry this video is sort of long I couldn't find any other footage of it on youtube, Skip to 3:37 to hear what its about or 4:42 for the scene): [vimeo]8450426[/vimeo]
First off, it's nearly impossible to take any sort of drive/ride/fly-by and use it to loop because of the doppler effect. Unless the passby is REALLY slow, you're not going to get enough material at the same pitch.
For looping, the sound needs to start and end in the same place, pitch and volume wise, or it'll be noticeable. For the sound of the motorcycle, your easiest bet would be to record about a minute of the bike in a stationary position, at the RPM you want, and then loop that.
As for the Reedman scene, I'd made that a separate post, and specify WHAT sounds you're talking about. There are A LOT of sounds in that scene, not just individual source elements but a lot of little things happening within the scene. Not to mention that they explain a major element in the scene already. Also, a minute:second mark of where to jump to in the video would help.
Just a guess, but it sounds like they may have used a Kyma to manipulate the magnet recording.
Actually I think that they simply put several of the buzzing-magnet sounds one after another. Maybe they used some delays to have more of a blurred sound. Because the magnets are mixed with other sounds they seem to melt into one continuous sound. ( plus that modulated delay kinda consolidates the sounds ).
Besides that I have to +1 Dave's answer