Yeah, it does almost sound like a loose rim, although I have no clue how that would have happened (unless you have an asshole around you who is loosening up wheels). I had this happen one time (different car) and it sounded just like a flat tire. Hopefully it's something simple, but in the meantime I'd find the lug wrench and at least make sure it's not the lug nuts (since that could turn into something really bad really quickly.
For reference, these are lugnuts (the 5 nuts surrounding the Scion logo in a star pattern):
and you should be able to find a toolkit with one of these in it:
(not exactly sure what the FRS one looks like, but it should be somewhat similar to that)
If you can tighten any of those lug nuts AT ALL with that wrench, it was loose.