I have contacted ARC service and according to them that the new KT120 tubes draw more current than the original designed 6550 tubes which caused the power transformer overworked and ran hotter and eventually making noise.
Since I did not buy my VS115 from an authorized dealer, the best they can do for me is to offer me a 50% discount on a brand new transformer. Also, they recommend me to use 6550 tubes instead and even offer to send me a new set of 6550 tubes for free if I buy the transformer from them. What a sweet deal and best service from ARC!!!
Next I will confirm if the 6550 tubes make the transformer run cooler and whether it will sound as good as KT120. So, please stay tuned on this. Thanks for listening.