That annoying sound you hear is T-Mobile CEO John Legere cackling madly. PCMag, which every year conducts a comprehensive study of wireless network speeds across the United States, has just published its latest study that shows T-Mobile has surged past ATT and now offers the second-fastest 4G service in the United States behind Verizon. This is quite a reversal of fortune for both companies from last year, when PCMag found that ATT had the fastest 4G network in the U.S. while T-Mobile didn’t even have enough LTE coverage to warrant a score on PCMag’s Mobile Speed Index. What accounts for this dramatic change? PCMag says that it credits T-Mobile’s aggressive efforts to upgrade LTE capabilities in major U.S. cities. “T-Mobile provided the