This guy retired a year ago but his site is still up. IMO required reading before you go spend a bunch of money on "mat". I've used his methods on my vehicles with great results.
https://www.sounddeadenershowdown.com/
"Mat" is not sound deadener, it's vibration dampener, and as an earlier poster said, you don't need to slather it all over your metal. 30-40% coverage is enough to achieve the goal of reducing vibrations..
What's more important is actual deadening material, meaning mass loaded vinyl. It's what they use to create soundproof rooms. It's heavy as hell, but actually blocks sound waves from traveling.
If anyone here is in touch with Don, the former owner of that website, please DM me. I've been trying to reach him. All good, I want to give him some money.