Check engine light and soft humming sound

Hi everyone,
I'm a new prius owner (just 2 months since I purchased my 2009 Prius Gen II).
Since two days the check engine light is on and I would like some advice about what to do.

I bought the car after a complete check up that didn't evidence any problem. The previous owner treated it with care: clean title, all maintenance record available and no serious maintenance intervention (12v battery changed on February and water leakage a couple of year ago). I must mention that since i bought it the low tire pressure light is blinking but the owner and the mechanic told me that it must be connected to a low-battery status of the sensor more than the fact that the tire is actually deflated.

Since I bought it the car run smoothly and with good mileage around 400 miles for full tank.

Nevertheless, since two weeks I'm experiencing some issues. Exiting from a mall, I stepped on the corner of a side walk that made the maintenance required light to appear. Nevertheless, after a complete check up of the car that didn't reveal anything the light has been turned off. Just yesterday instead a fixed check engine light appeared out of the blue and with no particular reason. I connected to the fact that during the last refueling I didn't tighten properly the gas tap. I also checked the air filter but since it appeared clean I decided to bring the car for the annual oil change and ask for a complete check up. Even in this case nothing new was revealed and the light was turned off. The repaired shop provided me the error codes associated (ph1121 and b4131) but googling them i couldn't find anything. This morning the light appeared again but for some strange reason the low tire pressure has stop blinking.
I noticed that also that the engine noise is accompanied by a soft, fast and more or less constant in frequency ticking sounds. I also noticed that the car is driving with a lower mileage more or less 5 mpg lower compared to my standard average. Also three days ago, while driving uphill i noticed that the engine was making an additional sound similar to one of sands bouncing at high speed but it disappeared afterwards.

Does anyone experienced anything similar? What would you suggest?
I'm convinced I need to take the car to a good mechanic, either the toyota dealer or a mechanic specialized in Prius. I live in Pasadena, could anyone suggest a good one?
Thank you in advance
Davide

 

Thank you for the answers and the explanation, dolj and bisco.
I have an update regarding the ticking sound, I believe its connected to the wheel: I realized it travelling with the window down and the sound comes from the front left side of the car. I checked the wheels pressure and they were according to the requirement.
I was thinking it was the carter that it is missing a screw connecting it to the body but it should produce that sound only at high speed while I feel it more at low speed. Do you have any idea about what could it be?
Thank you again
Davide