Rattling Noise under rear seat

I have Toyota aqua 2012 model imported from Japan mileage about 82000 Km.

There is no error sign or indication on the display but when i put the car to ready as soon as the engine starts a rattling noise starts coming from the rear seat and im afraid to drive further and have no idea what to do...everything else seems to be fine.

A few days back i just noticed then when i was pressing the gas pedal, there a slight delay in the car's rpm coming down. Even when i pressed the pedal hard and push back, the engine rpm (and sound) didnt come down as the engine kept reving. Went to a nearby technician. He told me that aqua has a sensor for gas pedal and he just took the terminals of the 12volt battery off and restarted the system.

Now it had lead me to the centre of road worried about what to do...any help?

 

With a heavy heart i managed to take it to my mechanic. He just overlooked it and suggested that nothing seems wrong. Asked me to come in the morning to have a detail checkup which includes removing the seat and looking for any loose electric circuit. Just an hour before i went again to the car with a torch and my camera to record the noise and see any issue. Apparently the car started and engine started when the battery got low and charged the system and nothing wring appeared

 

It was around 8 30 Pm so he asked me to come at 9 am to have a detail check up. An hour back i started the car to see any issue but nothing wrong appeared. And i checked the exhaust under the rear guard and it was working as i felt air brushing back of my hand through the vent. I also checked the

 

<9000 miles and already having an engine/electric motor issue | PriusChat

i think the description by the member is aced to the extent of my issue with the car. Engine revving up. Drop in mileage and finally the noise. I have cleared the vent too. Most probably the car was under my younger brother when all this happened and i got to found the plastic sheet rolled up near the vent and the oil can in a plastic bag on the same side as the vent caused the problem...i hope it will be resolved!