Does it change as you push harder and or let off the gas pedal? Kind of a high pitch squeal? If so you can try a few things to lessen it more. One by making sure the grounds connections are paint, dirt, and corrosion free. Two solder your connection. Last try to make the ground leads as short as possible.

And I know most people say crimp not solder the connection in a car because of vibrations, but I've never had a problem with solder connections and it has made night and day differences in my car installs.