I've got a sound I would describe as similar, although it happens while accelerating through a hard turn. It especially happens if I hit small bumbs, like a rough free way on ramp, if the front tires hop at all. It's more like a 'clunk', feels like its right under the pedels, maybe a little in front, maybe more in the middle, but I can feel it through the pedals.
What's up with this brisk reverse and hard braking, trying to shoot pebbles back out??
Mine definately isn't a pebble...maybe more like motor mount?