My Car Hits 2,000+ Revs In Neutral?

Just recently brought the car and have been driving it about abit and have had no problem with it at all..

But for the past 2 days now it has been going to 2k revs sitting in neutral or when ever i put my foot down on the clutch..

The car is a manual and has a air filter put on it to give a better intake... But i have no clue at all why this could be doing it..

When i go into reverse it makes an awful crunching sound because theres excesive over-reving..

Any answers will be good cheers!