I've been plagued by a buzzing coming from under the dash only at certain RPM and/or certain bumps in the road. Drove myself nuts trying to reach under dash while driving. Not recommended. In desperation, while the car was parked, using a rubber mallet, I tapped around under the dash and found the problem. The shell in which the OBD connector is located is covered by a piece of plastic that extends from the under-dash plastic shield. The vibration was that plastic interacting with the top of the OBD connector housing. A simple adhesive chair leg cushion slipped into the space stopped the buzz. Nice when things get solved.