If your servo is a 5645 that would be a normal digital servo squeal. If it not then I think you have a problem. Does it make that noise when it's not hooked up to the pull-pull wires? If yes, its a bad servo, If not then it's one of two things; 1. Excessive friction or 2. The geometry of your pull -pull linkage isn't right.

Oh yea, make sure your tail wheel is not sitting on the ground...