COULD IT BE THAT YOU ARE NOW ENCOUNTERING THE INFAMOUS RACK AND PINION PROBLEM that has plagued a notable number of Priuses!
You should most definitely take it to the dealer immediatly to assure that your issue/complaint is recorded before the expiration of the vehicle's 3yr 36K warranty period.
In my case, my rattle was initially diagnosed as a due to bad tires that were due to a bad alignment. I paid to have that done but the rattle persisted. I then told that it was my repaired muffler (paid @$100 to mend a pipe - outside of toyota), then my strut mounts took the blame, then the take was that there was no problem. Then no reproducable at the time the toyota factory service rep investigated the problem (and it was a long wait to have that looked at).
In the end, I had two arbritrations wherein the toyota motor corporation representatives adamently and slyly conveyed that nothing is wrong with the vehicle and if there was that they be allowed to correct any WARRANTABLE condition that MAY exist. There is an awful lot more to this story. Check your tires, you may find that the driver's tire wears abnormally at the outer edge. The tires at that position are rotated to the opposite side rear. You very likely now have other that the original tires somewhere on your car. If they are not the exaction version as the original then that will be the facet that toyota will seek shift blaming - your for not conforming to the maunfacturer's recommendations. Don't even think of mentioning that you may have used an inflation pressure other than listed in the owner's manual (even the use of an analog air pressure guage as opposed to a digital one was even offered up a challenge to my claims).
Also, checkl your steering wheel while driving on the highwaywhere I drove most). Mine is slightly cocked. Yes, there were numerous attempts in and out of toyota to address that. I have documented in writing and on video the dealer and non dealer attempts to address the alignment and steering problem. The abritrator chose not to look at that piece of evidence. They persist for they apparently do not significantly affect the value and safety of the car. If anyone else has run into this blockade, do tell.
WITH this general knowledge, you may be able to back the factory into replacing your rack (should you slightly slip past the 36k mile before it was conclusively diagnosed), aligning your car, and replacing your worn (if they are) tires. Without this knowledge, and their clear understanding that you possess such knowledge, it would be interesting to see how forthcoming they(dealer adn toyota factory rep.) will be to you.
Peruse this and other usergroup (yahoo or
http://www.corolland.com/prius/archive-toyota.html) for information related to the rack failure. Consider that here in NYC the vast majority of these vehicles I see owned by a governmental agency. I doubt the driver's of these vehicles are as attuned to them and to their maintenance/problem scope as individuals.
So, if you bought this used, you could be befalling a known problem that toyota is not stepping up to the plate to address but will hope to get away with due to the glare of the next generation prius.
I had a sign (temporarily removed it pending a respectful response time from intervening resources) prominently posted on my vehicle relating the tire eating, failing rack, and cocked steering issues that the unspecting should be made aware of. In addition, I indicated that I would not but the car again but should you like it, you can make me an offer.
The greater matter is customer respect, and committment to a written committment to toyota owners. Love can be blind, lead to infactuation, and allow the strong materialistic persuit and see no evil adoration of this interesting and promising vehicle to sinfully challenge values.