It seems to be noticeable at low speeds (0-30mph) and most pronounced between 10-30mph. It may still be there at high speeds and muffled by the sound of road noise. Not sure.

Originally I thought it was coming from the right wheel area (relative to sitting inside the vehicle) and chalked it up to a bad CV joint. Yesterday as I was replacing the headlights, I had to remove the bumper cover. While I was down there, I stuck my head back and checked out the CV axle boots. They look just fine. I supposed it could still be the CV joints, but thought it more unlikely at that point.

So tonight as I was driving home from class I tried to pay extra close attention to the sound, and it seems to ACTUALLY be coming from a lower area toward the center of the bay - around where the engine meets the transmission. Also noticed tonight: instead of sounding like a knock (which is how I originally described it to myself) it sounds more like a rattle - oddly similar to the heat shield rattle from the accord my brother has which is missing a heat shield bolt or two.

Thing is, the exhaust manifold heat shield on my car not only has all bolts, but they're snug and it isn't loose. Anyone have any guesses?

EDIT to add: It doesn't matter if the clutch is engaged or not and it doesn't matter what gear it's in. It doesn't effect the sound in any way.

 

I'm checking the exhaust heat shield tomorrow as well as the catalytic converter. Any more ideas?

 

The converter heat shield is the prime culprit. Mine rattled and seemed to be on there tight Upon close inspection, I noticed a thin crack through a bolt hole on the heat shield to the edge with a tiny bit of rust.

I would suggest stopping by a Lowe's or hardware store and get two hose clamps for a dryer exhaust hose to hold down the heat shield.

 

Checking it as soon as possible. I couldn't get under there safely yesterday because I have no jackstands or ramps where I'm staying.

I noticed that it doesn't happen until the car get's warm. Must have something to do with something expanding. After about 15 minutes of driving it will start.

 

Upon further inspection, there is indeed an exhaust rattle, and it's coming from the rear (although it sounds like it's coming from the front if you're IN the car). That, however, isn't the problem. The issue is the other sound.

Today I got out and pushed down on the car starting with the driver's side. I was able to reproduce the sound, but not much and it was coming from the passenger side. I moved to that side and again pushed down hard on the car. The noise was produced again, better this time. It happens as you're pushing the car down and as it's coming back up. Think the sound you hear when someone jumps on a spring bed or an old trampoline, only slightly lower pitched and louder. Those two things have one thing in common - springs. So that leads me to believe it could be the coil spring, but I don't know for sure and it was too dark for me to see anything.