Oops, I meant to update this thread earlier, but I guess waiting was better so I can give the good news

First of all, just using ferrites didn't solve the noise when plugged in to my car charger.

But, as jellotherat suggested, a ground loop isolator worked perfectly! Crutchfield has one that's inexpensive and has 3.5mm m/f so it works with regular audio cable:
http://www.crutchfield.com/App/Produ...=noise&tp=2653

I had both a low buzz and a rhythmic thumping and assumed the ground loop would only get rid of the buzz, but I'm glad to report that it got rid of both. Now I can charge my n810 in my car. Of course, I don't think it will come in very handy for a while since I can't leave it alone so it ends up charging at night when I'm at home

Hope this helps anyone with a similar problem!

- Andrew