how do I fix my mk4 1.4 16v astra??????

I have a vauxhall astra on a 1999 1.4 16 valve! The car was running fine n then it started t blow air out from sumwhere, n it wudn't rev over 4thou so I kept runnin but never over 4thou n then I went up a hilk n it dint have any power t get up the hill, I then recently cleaned the cat out n now it is tickin over fine, but is still makin that blowin of air noise n is also only revinn to 2thou please please help me cheers kris