Knock, Knock......KNOCK,KNOCK,KNOCK,KNOCK,KNOCK

well this totally sucks .I was on my way to my sister's house today with the family when my car started making this loud knocking sound,it knocked faster as the rpm's grew and got louder the longer i drove it.I've decided not to drive it anymore until I can find out what this is,but needless to say things are really looking grim for old Michelle(my car). :(