98 escort zx2 question

hello i have a ford escort zx2 it runs awsome not a prob there but ever since i got it the engine makes a buzzing noise. when i rev it gets louder and i have no idea what it is and cant pin point where it is. all the fulids have been changed no leaks trans shifts fine it even does it in park. can someone tell what it is??

Remove the engine belt and start it up without the belt for a short time to see if the noise is coming from a pulley or accessory.