Hello, I've noticed on my girlfriend's 2005 Prius (not sure of trim, it had a JBL sound system so I'm guessing IV) that when driving straight at highway speeds (esp 60-65mph) there's a slight rough/grindy sound. It goes away when taking a curve even if maintaining that speed, a fairly small turn of the wheel and things go silent and smooth other than road friction. My guesses would be steering column or wheel bearings, but thought I would check here before taking to a mechanic and coughing up dough. Thanks!
Use your hands to try to turn/shake one of the front wheels back and forth to see if there's anything loose in the steering rack... Also give the tailpipe a good shake. There's lots of thin metal shielding for the exhaust system that's prone to rattling down there.
