Tebop, In the past I have done what you said in your last paragraph. I stuck a microphone out my back door and recorded ambient city noises for as long as I needed for my animation. It took less time in the long run than trying to get clean, realistic sample loops. Another option: rather than repeating one sound clip, obtain several different samples of the same sound and assemble/blend them in random order. It makes the repetitions less obvious.