Second gear is usually the quietest of the gears so there is something amiss here.
Check that the hub is properly tight in the frame and that the rear wheel is aligned properly. From what I've read there can be problems in keeping the rear wheel properly aligned as pressure from the chain can can pull the wheel out of alignment.
Is the axle bent?
If that's ok, try and put the bike on a repair stand and turn the pedals in the affected gear. If you haven't got access to a stand, upturn the bike on a garden table outside and do the same. Spin the back wheel up with the pedals and listen carefully to the hub, while looking at the rear sprocket to see that all looks correct there. If the noise is coming from inside the hub, try adjusting the cable according to the instructions to be found online.
I adjust my srf3 by putting the handlebar changer in third gear and then adjusting the gear cable until it is just VERY SLIGHTLY slack - only a touch though. Then I try the bike out and make sure I can still get first gear. If I can't, I tighten the cable bit by bit until I can and then check again that I can get third. When all gears are reliably selectable, I tighten the cable lock nut.
The only time I've had a funny noise come from inside the hub was when I had the cone adjustment wrong. This can alter gear alignment. From your description though, I can't imagine it could be this.
There are good descriptions online of how to adjust the cones - but only mess with this IF YOU HAVE VERY GOOD REASON to suspect that is the problem. If you rock the rear wheel side to side, there should be only just a touch of lateral play. You can get internal noise if there is too much or too little because the gears can run out of line. This is a very destructive situation for those tiny teeth on the sun and planet gears and the gear ring.
Last edited by EvilV; 05-05-08 at 11:19 AM.