The "B" Screw, an often forgotten about adjustment and rightfully so. Hopefully this screw goes unnoticed to those who struggle with the rear derailleur adjustments. There’s a good chance this screw will not yield the fix your looking for. The only time this screw is turned is when the derailleur is first installed and even then some bikes have no reaction to it from what I’ve seen. So don't sweat this guy too much. If there is a problem you should notice it in a particular gear, the biggest gear in the rear (cassette) and most likely the smallest chain ring in the front. This selection will allow the chain to relax quite a bit producing a problem IF it exists, not always.
The problem is this, a noise that is produced when upper pulley wheel of the rear derailleur is so close to the biggest gear on the cassette, it begins to rub. The pulley wheel has the chain riding on it, which, together, makes contact with gear, producing a rolling vibrating sound. Simply* turn the "B" screw clockwise (righty tighty) enough to allow some space between the two. Textbook calls for 4-5mm of space. *In some cases the screw is hard to turn so you will have to unweight the derailleur body along with the "B" screw off of the hanger, taking the pressure of the screw, allowing you to *easily turn the screw.
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