I know I've seen this before on these forums but I couldn't find it and it's really starting to bug me.

There's a pretty loud clicking sound coming from behind the passenger seats. At first I thought the electrical connection to the defrost lines came loose and was slapping against the window (that's what it sounds like) but I checked and it was solid.
I think I remember this having to do something with an exhaust problem? Although that makes no sense to me it's extremely annoying and I'd really like to get rid of it.