Originally Posted by Jafro

Those are air brakes. You'll hear them after they take their foot off the brakes, and after they stop.

Most deisel engines red-line around 2000 rpms, and have about a 52:1 compression ratio, don't need plugs because they run off of detonation, and have rods the size of a cow's leg bones. BOV's aren't necessary. Turbos for diesel engines are gigantic in comparison to turbos for gasoline engines and with the low RPMs, they don't see a lot of abuse.