there are so many different ones!
the cheapest one i found was 43$ (behringer uw300)
the most expensive one was 623$ (z.vex ooh wah)
but i can recommend a dunlop cry baby for about 110$ =]
That sometimes happens when there is a cable or wire that is not shielded correctly (That outside metal mesh in the wire) or bad grounding of the equipment. Bad ground can also give you quite a...