Diesel vans can be quite a bit louder than their gas-powered counterparts. Though engine noise will vary by make, model and year of the van, many owners say they have to at least turn up the radio, and in some situations have to almost yell over the noise. There are several options available for muffling or preventing cabin noise. It's also recommended to put the car on cruise control at higher speeds for long car trips; this keeps constant revving from interrupting conversation.
Check the resonator and doghouse (engine cover). Most rattling or excessive engine noise can be attributed to bolts which aren't tight enough on the doghouse. Pop the bonnet and make sure the bolts are secure. If not, tighten them completely with a ratchet on both sides. The engine cover should be extremely resistant to movement of any kind. If the resonator has been removed for convenient access to the engine, this may also contribute to your noise level.
Use sound-deadening mats. Available from auto parts stores and hardware stores, sound mats are thin sheets that dampen vibration and reduce road noise. Many are self-adhesive and require only scissors, rags and rubbing alcohol to apply. Using a roller may ensure that they go on more smoothly. Ask your salesperson about working in patches; you may not need to cover the entire car. One manufacturer suggests starting with the doors then moving on to the rear, floor and roof. Some mats also help keep heat inside the van.
Use rigid insulation or rubberised paint. If the sound-deadening mats are out of your price range or too difficult to install, use a spray adhesive with rigid insulation. You'll want to make sure both the adhesive and insulation are waterproof; otherwise you risk built up condensation "showering" you at every traffic light. Start applying the adhesive to the doors and side panels, and add more as you see how it affects the noise. Another option is rubberised paint, often sold at marine supply stores. Initially designed for boats, this sound-deadening coating can be rolled or sprayed on. Start with a coat of primer, then add the paint evenly and let dry.
If the noise is coming from below, consider adding marine plywood under the rubber floor mats. Pull up the mats, then secure waterproof wood and adhere the mats to that surface. In addition to reducing noise, this will help your vehicle heat more efficiently. If you're unsure how to add the plywood, consult a local repair shop.
Raise the roof to reduce wind howl. If your cabin noise isn't reduced by any kind of repairs or muffling, your main problem might be attributed to wind howl. This is likely if the noise increases at fast highway speeds. Some car owners have found that raising the roof reduces this significantly. However, it's a costly procedure and may take time. Therefore, use it as a last resort.