Ford E-350 Problem Report
Ford E-350 Buzzing Noise from Exhaust at Different Engine Speeds
If a buzzing noise is coming from the exhaust at different engine speeds and conditions, this is commonly caused by the heat shields on catalytic converters coming loose at points. The buzzing is the metal on metal contact. Installing a large worm clamp around the catalytic converter and heat shield assembly will prevent the noise and secure the shields.