Does anyone else's phone vibrate consistently every 3 seconds or so when it's charging while powered off?
Yes! Mine does exactly that and it's extremely annoying! I hadn't noticed until today. I started running CM10 as of last night, but that might be a coincidence. Are you still using stock ROM?
and it's still happening when you charge with the phone off?
Does your LED light up as a charging indicator? Mine does not anymore.
A friend of mine who's rooted, but still on stock rom says he gets a red charging indicator when he turns his phone off and plugs in, but no buzzing.
I had to use stock rom yesterday so I tested charging with the phone off and there was no strange buzzing. I didn't get an indicator light, rather the scree itself stayed on with a battery charging graphic. Regardless, I'm pretty confident that the buzzing when charging with the phone off is symptomatic of this early version of CM10