When i start my 2000 GT other than my dumb exhaust leak (getting fixed tomorrow) there is like a rattling/ knocking sound. im pretty sure its not a valve tap as it doesnt happen the entire time car is on, just when i get the rpms to about 2500-3500 then there is nothing. If my car is in a high enuf gear i dont even hear the noise, a friend of mine and a mech, said that it is either a valve chain, cam, or push rod. im hopein its just some little bs. the car was all fine until Sunday when it first started, and i dont know where it came from. so can someone help my dumb self out wit an explanation of what these problems are and which if any i should be rooting for lol. car has lil over 125k miles. any input will be appreciated thanks!