Banging on startup

Just bought a 2010 (after some kid wrecked my 2000 Mystique; that's another story). Not long after I bought it, something started banging on the right side when the engine is started. Bangs for maybe five, six seconds; then it stops. My wife has occasionally gotten the noise to stop by pressing on the cabin air filter cover. But the kid who sold me the vehicle guessed there was something wrong with a blend door. Moving the HVAC controls has no effect on the banging. Any ideas, folks? Thank you all.