I have the same issue. On acceleration and deceleration I hear what sounds like a small rock, rolling and bouncing underneath the driver's seat. I'm guessing it's a rock, or at least something non metallic.
I haven't been able to find a way to access the area where the sound is coming from. There must be a cavity between the seat pan and the undercarriage that isn't perfectly sealed.
I've had something very similar for about two months. Good news is that I've solved it!
In my case it sounded like something rolling from side to side. I would hear it on turns and my guess was that it was something behind the driver's seat. In my mind I imagined that it could have been a small part possibly made of rubber. I'd also sometimes hear it when I accelerated a bit faster than normal.
1) I checked under both seats - just a few small rocks that I cleared out.
2) I found a small candy on the aluminum tray to the right of the steering wheel. I was exctied by this find but it was not the problem.
3) I removed the undertray and found nothing.
4) With the undertray removed I looked inside the area behind the driver-side sill. There were just a few tiny stones.
5) With the undertray off I noticed the aluminum plate with all the 3-4 inch diameter holes in it. I couldn't get my fingers into any of the holes so I gave up inspecting that part.
6) After I put the undertray back on I noticed that I had not looked anywhere under the muffler in the area of the diffuser. So I slide a camera into the diffuser area and took a few pictures.
The areas I wanted to check next were:
- Inside the driver-side sill. I planned on removing either the driver-side sill cover, the air duct slats or the rear wheel-well liner.
- Inside the plastic housing that holds the rear speakers.
Before I started to rip apart more of the car I took the photos from 6) above and looked at them on my HiDef TV. I noticed that there was a fairly obvious stone present. It was on the passenger side of the diffuser.
Hear are some photos.
A) Looking into the bodywork on the driver's side. The area is right under the side air duct. Stones in there would eventually bounce out onto the undertray.
B) The same thing but with more detail shown.
C) The aluminum cover with the holes in it.
D) A stone in the diffuser.
After removing the stone in the diffuser my "rolling object" noise has disappeared. I think the sloping angle of the diffuser cover had kept this stone trapped there. Also the green bolt (in picture D) exists on each side of the diffuser. I think the stone was sliding between the two areas in which there are green bolts. If the stone had been a bit smaller it would have just disappeared through the gap onto the undertray.
Note that in my case the problem was in the diffuser area but sounded like it was either under the driver's seat or immediately behind it.
I am REALLY happy that this noise is gone. Drivnig the car is much more fun now.