Hum/buzz in recording

There are so many possibilities with such a little information that it's hard to know where to start, but there are a few things you can try (and have sort-of inferred...). If you plug speakers in, does the same thing happen? Does it do this on all files you've recorded, or are earlier ones not affected? One thing we could check out here is whether there's any indication of this being a recording problem - place a sample somewhere, and post a link to it.

Other things: if you plug the headphones into anything else, does the same thing happen? If you move the lead around, especially at each end of it, does it alter?

That's probably enough for now, but I'm sure you get the idea...