I had the same problem with my civic. It happened to be the brake fluid resevoir cap. When I removed it to do the brakes, I replaced it and the reving problem went away. I then took it off and put it back on, and the reving came back but only for like 20 sec. I did this a few times and the same thing, I thought it was strange, but just figured there was some kinda vacum on that system too.