Well, I had the same problem on my 00 C70. I took it to the dealer (cuz I THOUGHT I was under warranty, apparently this didn't count for that). They told me I needed a new headlight assembly. I didn't want to pay their 300 dollars for one so I went off to find me own. I found an OEM assembly for about 180 from eeuroparts.com and went ahed and started installing it. When I got the old one out I took a look to see what was shaking, apparently a small plastic piece on the back had come unhooked from where it was supposed to be, like a little upside down U piece that fits over a small sphere piece that holds everything down tight.

All I needed to do was to push this back together and the shaking was over. However, I already had a new assembly installed and I needed a new lens anyway so I just left the new one in.

Anyway, just pop out the assembly (it's easy to get out, so I wouldn't bother trying to do it while still installed) and then look behind the bulb covers, you should be able to see what I'm talking about pretty easily.

Good Luck!
Scott Silverman
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R.I.P. - Wrecked in Hit and Run --> http://www.stleasing.com/scty_boy.htm <-- '98 S70 T5-M ***** '00 C70 HPT 5-speed, NBM/Beige Full Leather, 400w Dolby PL