I noticed this the last couple days.... Sometimes when I open a door (from the outside of car) I hear a weird noise that sounds like it's coming from under the car. I would put it somewhere between a 'click-click' and a 'clunk-clunk'. Sounds like something is engaging or disengaging? Main pack? I also heard it today when I plugged in my UMC. It doesn't do it every time though. I've never heard while driving, but I also usually have music up pretty loud as I drive mostly alone. Any ideas?