Mystery Sound on Mazda6i at 2500-3000rpm only when engine warm

Hi all, my 05 Mazda6 (147000 miles) makes a roaring sound when accelerating and hitting 2500-3000 rpm. It only does it after being driven fast (interstate) for about 20 min, so when the engine is warm. Interestingly, the sound disappears once I get off the interstate and have stopped at maybe two traffic lights. After the stops, it runs excellent again, as it does in city traffic/commute where I don’t drive faster than 50mph for an extended period of time. Even with the roaring, there is no loss of power. Recent services: transmission service, full tune up. My mechanic said he didn’t hear anything and the car looks excellent (probably didn’t drive long/fast enough)Any suggestions?