wacaidor

I just had a knocking noise start this morning. It didn't start when I first pulled out, but rather as I approached a red light and began to slow down, and has continued since. It comes from the driver's side and sounds like I'm dragging a piece of metal or something like a twig that's knocking against metal. The frequency of knocking increases and decreases with speed, but isn't influenced by turning. There is nothing obvious stuck on the underside of the car, in any of the wheel wells, or under the hood.

I know that one of the tire pressure sensors came loose in one of the wheels two winters ago (I can't remember which one), but that one didn't make any noticeable noise. Could that still be it?