Humming in some of this units is a very common problem for people. After reading about the use of shielded wire for the signal path from jacks to switch and switch to PCB helped on taming the humming noise, I gave it a try as well as checking that everything was grounded properly. By the words of the owner, the problem was solved. Before it was humming even in bypass!

Original guts.

Input and output wires replaced with shielded wire.