This comes up about twice a month and, unfortunately, the answer is pretty much no. There is no way to magically enhance good sound and reduce bad. That's why TV shows and movies frequently "loop" or dub in audio for scenes shot in noisy or windy conditions.
About the closest you can come to this is, if the sound has a specific frequency (like a hum) you can sometimes filter out that particular frequency. But even that can give your audio an unnatural sound and can even accidentally remove some "good" sound too.
I know of one effect that has helped me out quite a bit to lower unwanted noises. Simply called "DeNoiser," I have used it many times and it has worked well when I tweaked it enough. Just be careful: Put it on too heavily and it may make the sound even worse...If it works like Pre 8, just go to the effects panel and choose audio effects--it should be there.
As I say in my books, the primary use of the Denoiser is to remove tape hiss. It does this by filtering sounds of a certain frequency. As I said above, it can't magically separate good sound from bad. And, as Infinity points out, setting it too high can make your audio sound very artificial.
But it's definitely worth a try!
As has been stated, repairing background noise is tough, and depending on the noise frequencies vs the frequencies of the narration, can be impossible. This ARTICLE gives some tips, but they are just that - tips, and can require a lot of work, with possibly little effect.