Mine, and all of the others that I have sold operate very smoothly. If the compressed air trick doesn't work, take the phone back to a VZW store and get it replaced.
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LG VX9900 (enV)
BlackBerry 8130 Pearl
Defect in the tracks usually, ive delt with allot of these phones in selling and repair. The tracks are installed crooked in about 10% of the models I sell. They make a scraty metal noise.