I'm surprised nobody has mentioned mass loaded vinyl.
According to some, it is way more effective than dampener sheets.

Unfortunately these guys are out of business, but their website still has a great discussion & comparison of the different sound deadening strategies

https://www.sounddeadenershowdown.com/products


Mass loaded vinyl is what I want someday... it would be interesting to hear if anyone here has tried it?

'59 Chevy Suburban, NAPCO