Help! odd new noise

I have a 2003 prius with 184,000 miles on it, and it just started making an odd noise. When I start off from a stop I will hear a knock or tap sound that appears to be comming from the right rear roof area. like I kicked up a tiny stone or a nut falling on the roof. It is usually a single knock but sometimes it's a quick series of three or four knocks. It occurs between 23 and 29 mph and usually from a full stop but occasionally when braking to below 10 mph and accelerating back up. I've checked out the suspension, brakes and everything in the trunk area and found nothing. Has anyone else encountered this or have any clues? Thanks

 

Thanks for the reply, but I don't think that's it. I have no hatchback. My car is a sedan. I couldn't find anyplace that delt with gen1 prius, so I posted here. Here's an update though; it's been a couple of days and with another 300 miles the noise is now louder and occuring at both lower and higher speeds 5-35 mph and also while cornering. It does not seem to do it if you floor it during acceleration from a stop though. Is there someting funky about the braking system that could do this? The only thing I can find posted commenting about a similer noise, happend after being parked a while. Comments were made about a brake accumulator? Never heard of one, what is it, and what does it do? Thanks for any help.