Strange static when I play back vocal recordings?

Ok so im using a windows 7 laptop hooked up with fast track pro m audio with a mic coming from fasttrack. When i record vocals and listen to them in Sonar x1 i hear a faint static more like a white noise when the vocal is being played. There is no static when the vocal is not played yet, its just happens when it gets to the voice.

Do I understand correctly: It's a static-type noise which goes in sync with vocal recordings and you hear it only on playback?
What kind of headphones do you use during vocal takes? My first thought was leak from headphones (which is so quiet it can be confused with noise).
Second suspect is the microphone. Do you hear the noise if you record guitar or other instrument direct (without mic) or micing your amp. Is there any difference whether you sing very close to the mic or from, say, two feet away (well, if it's a dynamic one it won't take anything that far)?

Is there maybe some FX used in the soundcard or FX bin of the track?

I heard something similar in a vocal track a couple of years ago (completely different hardware). Very quiet scratchy (white noise) that seemed to be modulated by the vocal. I think it may have been generated by an issue with the phantom power to the mic (speculation on my part). I had it on some vocal tracks but not on others, which makes me wonder if it was in issue with the preamp supplying the phantom or a marginal mic' cord.