As i exited the freeway i shifted out of 6th gear (70mph) and pressed the brakes taking it down to 40mph. the rpms continued to rev up to 6k and beyond. the brakes did respond, so i felt there wasn't immediate danger. i immediately pulled over to the shoulder and stopped my car by turning off the key. turning the car back on, not in any gear, not pressing the gas, the rpms began to rev just as high again. so i turned the key off again. i decided to try again, started the car, it began revving up the rpms. i shifted into first gear to see what would happen and it began to decrease in rpm. then it became normal. but now there is a knocking noise in the engine when i accelerate that was not present before. it's less noticeable at 1st and 2nd gear, but gets more pronounced from 3rd gear onward. there were no warning lights, no check engine light. i took it to firestone where they told me it's possible that the engine may need replaced. i did find there was a report of engine processing flaws that could result in a knocking noise on various news sites but nothing official. this car has only 40k~ miles. it was manufactured in 2011 as i bought it hot off the lot at 6miles on the odometer. i get regular oil changes using synthetic oil only. it's mint. no reason for this to be occurring.