I have a 2006 Scion XA. Few days ago noticed revving sounds while driving the car. Originally thought engine was revving but noticed RPM gauge is normal. The humming/revving sound starts about 20 mins after driving the car and sound will last 6 -8 seconds. It will continue on and off for about 5 -6 times and noise goes away. Car drives fine with no problem starting, accerlerating, breaking, etc. No dashboard lights are on. Brought in in to machanic and he could not find any problems with car. Any idea what the noise might be?

SOURCE: clicking, squeaking noise and rough-idle when parked in drive

It almost sounds like you have a heat shield that is loose on your exhaust system. Let it cool, and try to reach under, lightly bumping the exhaust pipes with your hand. If you hear any rattle noises, or squeeks then you have a loose shield. you can ither remove the shield, or try to use a large clamp to go around it and tighten it up.

SOURCE: I have a Scion xA

it can be low tire pressure or you just need an arlinement

SOURCE: Engine Knocking Noise

I don't think that the petrol quality will come up with this noise and also the engine is fairly new to have these sort of problems.. Have you run out of oil and used the car? I would suggest a computerised diagnostic if you are getting any error code on the dashboard.It is quite complicated to get an answer for your question...

SOURCE: my 2005 scion xa is making a loud scraping/

SOUND LIKE BRAKE PADS WORN DOWN TO METAL GRINDING ON BRAKE ROTORS.

SOURCE: VSC TRAC AND CHECK ENGINE

did you left your gas cap loosse, well every time check engine lite is on, your vsv lite will come on has well, if check engine lite is not flashing you ok to drive it but have some one scan it asap,j you dont want a problem getting biger,
good luck