Mechanical noise isn't unusual with transformers. The only real fix would be to "pot" the offending coil of winding. Do check to see that there is no voltage on the chassis or anywhere voltage shouldn't be though because excessive current can have this affect on otherwise well behaved windings. If there is no voltage where there shouldn't be and the unit operates with the same range it always has, I'd bet that the unit is fine. As long the noise isn't too offensive.