Clinking noise and sproing sound

Hello,

Today we heard what sounded like a flag pole clinking while driving in a parking lot. Turns out the sound was coming from my Prius (2010 III). We then heard several sounds, going from the clinking to what sounded like something falling off and something possibly dragging for a few inches. When we stopped to check, we found a large amount of pebbles resting in the belly (body? undercover? the black plastic parts on the bottom?). Tapping the plastic would cause pebbles to fall out.

My questions:
1) Is that what was making the sound, or is there something else we missed?
2) Is this common? Is there something I may have done to suddenly increase the amount of pebbles/gravel or to have caused whatever is making the clinking sound?

Additional notes:
1) Nothing was dragging on the ground when we got out of the car, though I'm not sure how far down the car goes with 3 adults in it.
2) I can only really hear it when going below 20 MPH.

 

Drove it to the bank and back and this time heard what sounded like a spring overwinding. Made a sproing sound.

 

Is this what I'm looking for? Not sure if the other message sent, but I heard what sounded like a spring over winding, like sproing

 

While looking for things, I don't know if it's only been doing it when making turns, I only know I hear it when accelerating from a stop. I'll listen again on the way home.

 

Didn't hear anything while leaving the parking lot, which had at least 4 turns. Didn't really hear anything on turns. Was missed evident while braking or accelerating at stop signs and lights. Did hear the sproing again, too.

 

And then I heard nothing from the gas station to a half mile home. Seems to be on and off, so I'm not really sure what could be causing it. All I can say for sure is that I don't hear anything above 20 mph.

 

Dealership wouldn't be able to look at it till next week, and at $94/hr, so I'd rather avoid that if possible. @_@ What springs and coils are there that exist that could potentially cause problems?

 

Edited title now that I'm sure it has nothing to do with the pebbles.

 

All right, so I took the car out to test it on my own. I heard absolutely nothing regarding all this. So now I'm putting myself at two options: 1) it was just rocks bouncing around (which would create the clinking, and if big enough to hit the springs, a sproing sound) and now they're all done (not that I would know where they really came from, I don't think I did anything crazy today in gravel); or 2) there's something wrong with the shocks/whatever and it can't support three overweight/obese people, which would kind of make sense since it was coming from the right side where two out of three were sitting, and would make sense as to why I didn't hear it when only I was driving. I'll be test driving it again in about 2 or 3 hours so I'll post back again after I pick up my roommate. Thanks to anyone who is dealing with my oblivious-ness.