If there is truly no binding in the flap or the linkage (I assume it's a straight line connect between the servo arm and the flap horn) then you might try to work the flap on both wings a while to sort of loosten them up a bit.  They may just be stiff.  You might be hearing a sort of buzzing noise as the servo pushes the flap against resistance.

You did say you are using old servos from an Alpha Trainer you totaled last year.  How certain are you that the servos are ok? 

With the cost of servos, heck, you can get a decent Airtronics 94012Z servo for something like $10.00, you might just consider picking up a pair and using them rather than the ones you may not be certain about their quality.  The 94102Z servos are just fine for flaps on the PTS.  I used them on flaps on my Goldberg Wildstick 40 which had full deflection in two stages.

CGr.