Earplugs or ear defenders - and I'm not kidding, either. In my last wagon we used flashing tape - which is essentially a bitumen-backed foil. It was pretty effective, but be warned that it is the very devil to remove.
You have to cover the whole of the interior bulkhead, footwell, floor and drivetrain panels and the seatbox, too, for it to really make a difference.
When we get the bulkhead welded and engine out on my latest wagon, I intend to soundproof the engine side of the bulkhead and then the floor panels and seatbox from the underneath. Inside I will use thick rubber stable matting held in place with long rivets and washers rather than glue ... and over that an old-style Hardura interior trim. Overkill? I certainly hope so.
