The key to soundproofing your car is understanding how sound gets from the engine to the inside of the car. Instead of travelling through the body of the car, sound comes in first through the bonnet, then through the windshield. The bonnet blocks very little sound and the windshield blocks even less. Applying a sheet of vinyl nitrile foam to the underside of the bonnet will drastically cut down on unwanted engine noise.
Apply several pieces of tape to a sheet of butcher block paper.
Apply the paper to the underside of the bonnet, taking care to line it up, against the bonnet.
Cut out any sections blocking air filters or other necessary bonnet features with a knife.
Carefully trace the outline of the bonnet onto the paper with a pen or pencil.
Remove the paper.
Attach the paper to a piece of vinyl nitrile foam mat..
Use a sharp knife or an electric meat cutter to cut the mat to the shape of the paper.
Remove and discard the paper.
Clean the underside of the bonnet with a cloth and rubbing alcohol.
Cover the engine with a cloth to stop cement or debris from falling on it.
Spray contact cement on the underside of the bonnet and the piece of mat.
Carefully attach the mat to the underside of the bonnet.
It helps to have a friend to hold the bonnet while you trace the paper and attach the foam. Alternately, you can remove the bonnet before you get to work.