It could be that your gear indexing is slightly out of adjustment.
Have you tried applying light pressure to your shifter (up and down) whilst peddling?
If your indexing is out, then applying light pressure will alter the noise coming from your rear cassette. If applying light pressure makes the noise go away then you will need to make an adjustment to your gear indexing by turning the barrel adjuster on your outer cable (either at the derailleur end or the the shifter end depending on you bike) a quarter of a turn at a time either way until the noise is gone.
Essentially, what you are trying to do is ensure that the derailleur jockey wheels are sitting directly over the gear on the rear cassette for the selected gear to ensure the gear is running through the centre of the chain and not rubbing on either edge of the selected gear or worse the adjacent gear. As cables and components stretch and wear over time the indexing will shift gradually, to the point where it needs adjusting.
I would start in the middle of the cassette, and once you have that dialled in, check up and down the cassette to make sure everything is in line. Top and bottom gear might be very slightly out, so a quarter of a turn either way at the extremes might be required, but if you can live with a little bit of noise in those gears I wouldn't bother.
Hope this helps.