I just did my 300CD with Dynamat Extreme (yes it is expensive) and the results are well worth the effort..The outside noise level has been reduced to well below what I thought it would be...Especially from the trunk where I really laid it on thick for the Sub Box. Worth the effort....

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83 300CDTurbo 307,000
2005 Honda Element 266,000
56 Nash Ambassador Country Club Special 34,000(under restoration presently)

http://www.peachparts.com/shopforum/showthread.php?t=125099