First make sure that your spokes are really tensioned correctly what might feel like good tension to you might not be enuogh. It could be sound coming from a spoke when it is loading and unloading as the wheel is turning causing a slight movement at the nipple rim seat and nipple threads and/or rubbing between that spoke and another in contact with it where they cross. If the tension on the spokes seem to be OK, maybe you can eliminate it by doing what wheelbuilders do when they do final tensioning on the spoke called "pre-stressing". Grab four spokes from separate sides of the wheel with you hand and squeeze them together just so that they all get a good tug. Do this all around the wheels a couple of time makeing sure you get all spokes. The pre-stressing might just move whatever's making a sound enough to eliminate it. If it persists, you can also give all the nipples a good turn and back with a spoke wrench, to further move things around a bit. Don't know what else you can do if that does not work that's short of having a shop check it out and try to find and fix any problems that might be causing it.
Chombi