I have a 03, agile off road conversion. When I first got it empty as a cargo van with zero insulation just a tin box it was crazy loud. Anything over 30mph you had to yell to have a conversation. I have since built it out and insulated the crap out of it.
Here is what I did
First I blew foam into every crevice and opening I could.
I then covered the whole van with rattle stop, this includes the dog housing,floor under driver, passenger, back floor, walls, ceiling etc.
I put in wood floor with the coin rubber garage flooring and got a ceiling from a cargo van. Did the walls with fiberglass insulation then wood covered with boat decking for easy cleaning.
At this point the van is quieter then my father-in-laws new dodge diesel. Its still not as quite as my wife's luxury sedan but its pretty damn good.