Hi Dave - I see you found the site.
This is Mike - WB9DLC.
I found this Crutchfield site that is a good troubleshooting site for vehicle noise. It also offers part numbers for various filters that might help. Also some good troubleshooting ideas.
http://www.crutchfieldadvisor.com/ISEO-rgbtcspd/learningcenter/car/noise_suppressors_installation_guide.html?page=4
I hope the long URL makes it through the system.
73