i have a 2007 ford focus 1.6 se hatchback, the idle goes to 2000 rpm then drops down to 900 rpm when i reach a stop street or when the engine is hot, there is a loud humming sound coming from the intake manifold, i had to leave a gap in between the air filter box so that the humming can stop but now its sucking air and makes a loud sound from the air box. i have take the inlet manifold out and cleaned it and checked for leaks and cracks but could not find anything wrong. I would really apprciate if someone could assist me with this problem.

Sounds like a big vacuum leak at the intake, here's a video of symptoms and how to find vacuum leaks: https://www.youtube.com/watch?v=qTzDY95aqXg&utm_source=FordOwnersClub&utm_medium=ForumLinks&utm_campaign=referrals