2000 Honda Civic makes a low tone sound while accelerating

2000 Honda Civic mileage: 95,000. I just recently purchased this Honda as a commuter car. When I first purchase the car there was and still is, a low sound when I accelerate the vehicle; it sounds like an exhaust leak. So they said they would repair it. After looking at the car, they determined sound comes from the air filter box. They got the diagram from Honda and everything is supposedly there. Now, they say it is perfectly normal. Do you know what this low sound could be that comes out of the filter box when accelerating?