Shadow SH-2000 Quick Mount Transducer with Volume and Tone Controls

I was bought one of these, which worked perfectly. So I bought another and at certain frequencies, i.e. when I plucked certain notes, it made a horrible rattling buzz on the amp, so I had to return the second one. Great product when it works, but there may be an issue with quality control. The volume and tone controls don't have any markings on them either to indicate which is which, and which way to turn them - difficult in a loud, gig situation. I had to write on them myself.