I am new to the sport and am putting together a trainer. I hooked up the ailerons and the servo makes a buzzing noise. Is this normal? What does it mean? How do I fix it or is it ok to leave it that way?

The usual cause of a servo buzz is a load at the end of the arm travel or something binding in the linkage. Make sure that all pushrods and control surfaces are free to move and that nothing stops the servo from traveling to the end of normal travel. Adjust your push rods or controls on the arms, in or out as necessary. All control surfaces should be free moving when the control rods are not installed.

On some of teh newer 'computer radios' you can limit the servo arm travel. that might help too...

What if it happens when the ailerons are in he neutral position, not at full stroke?

This could mean that the linkage isnt exactly right. Your linkage might be just a fracton too short or two long and when you press it down on the servo then the servo takes the abuse of the linkage not being lined up exactly right. Take the arm off of the servo and listen for buzzing, if no buzzing occurs then the servo is more than likely fine, and your problem (probobly minor with a trainer b/c it doesnt have a whole lot of stress as say a 40% does) will be in the linkage more than likely. This stress is not really good for the servo but I always say "its just a trainer". It would be a different story if you were running $100 digitals in there on a 1/3 scale but your not.

Take the arm off the servo and see if it still jitters. If yes, plug a different servo into that spot and see if that jitters. You might have a bad servo, electrical noise or some other problem.
Andy