I just got off a big job last week. She had 10 florescent fixtures with only one light per fixture, but several were humming.
I ran a check list on the components of a florescent light and there are not that many components. The bulbs were new, the starters if they had one, were new, and the only thing that was old was the ballasts.
I replaced all the ballasts as the fixtures were 5 years old. They had alot of money so they just wanted all the ballasts replaced. I thought it would have been cheaper just to replace the entire fixture, but for some reason they did not want that.
Anyway, the fixtures that were humming did not do it after the new ballast were installed.
There is a super site showing you how to replace the ballast. Jay the Moderator has the addresses. You do have quite a few wires that you need to properly connect up.
Try this on the hummer and see if it works. On florescent lights, it is very important to have a good ground and tight hookups.