Cody Oughton

Hey, maybe someone can help me out?! I feel a mechanics trying to rip me, so this is my problem I have a noise coming from back of my motor by fire wall(not any suspension) and it makes horrible grind at idle and only up to 20 mph. After I go over 20 the noise goes away? And comes back after my car drops under 20 mph. Again it's not suspension brakes etc. someone told me a flex plate or a torque converter some one help me please