This has been happening since the car was new. It's annoying. It doesn't do it all the time but my rear speaker grills resonate an annoying hum at low frequencies. I never seem to have the chance to check it out. But today while the music was playing I opened the liftgate and put my hands on the speaker grills and I could feel them resonating. Even when I applied pressure on them they still did it only less. Has anyone ever had this happen? Its the stock sound system. Its a 2006 Magnum R/T. Is there some way to stop it? Thank you.

 

this may be a dumb question, but is something loose, cracked?

 

No, there is nothing loose or cracked. I wedged some stiff type foam under the plastic and it reduced it a little bit but it still resonates. Isn't this a common problem? I really does it a lot on Black Sabbath's song "Planet Caravan", a mellower tune.