Keeps revving by itself

Hi, I recently bought a vauxhall zafira 2011 model. The car keeps making a revving sound when no one is touching the pedals. And the Rev counter keeps bouncing when the car is idle. I took it back to the dealer who said it was the diesel partical filter, they cleaned it as it was full of soot, they also told me to go back if the problem happens again. I took it back today and they have told me that they have had it out on a test run and also they put it on a diagnostics machine and said they can't find anything wrong with it. I am getting a second opinion, but I was just wondering if anyone on here might know what else could be causing this problem. Thankyou