> To listen, go to bostonglobe.com/soundtrack. You know how when you’re driving and you suddenly hear an ominous noise coming from the road, and you think, [expletive], why is my car making that sound? Anxiety floods over you: Is my axle about to snap? A tire going to blow? Oh, my God, where’s the breakdown lane? Usually, your concern falls away, because you realize it’s not your car; it’s the road. It’s supposed to sound like that. Such is the case in the southbound lanes of Interstate 93 as the snake through Medford. When cars and trucks zip across certain patches, the marriage of road surface and rubber produces an audible growl or whine. My first couple of test-drives, I was going about 50 or 60 miles per hour. At those speeds, the sound is more of a low buzz (imagine Luke Skywalker swinging a lightsaber in Star Wars). Later, I drove over the same stretch going about 70, and the pitch was higher, like a cellist bowing an A-flat. So what causes it? Accidental divots left by heavy construction equipment a couple of summers ago, state transportation folks say. The old Boston Garden floor had its idiosyncrasies. Now 93 has its own.