It seems to go back and forth. I need to get in touch with my ISP and see if they can figure out what's causing the noise. I'm getting ready for a trip in a couple weeks, and dealing with an ISP is a bit of a time commitment, so I don't really want to dive into it right now.
<edit> screw it, I just put in a support ticket with my ISP. If they don't get back to me within a week or two, or fix this by the time I get back from my vacation (exactly 1 month from today), I'm going to look into changing providers. Their service this far has been absolutely unacceptable.