If you haven't accidental hit the Cylinder Select knob, and the Red and green wires weren't shorting together, I'd say the Tach unit has Lost Frequency to analog converter, Or the meter is getting a high current source it shouldn't.
Check the select knob (if you have one..) and the red and green wires for shorts..If good replace the tach..(unless you wish to attempt to rebuild the unit..)
Spent the day replacing my Carter electric fuel pump with an Edlebrock 1791. A lot of work since the fittings are in different locations. The Carter sounded like a machine gun. The Edlebrock is a lot quieter.
I was surprised how much noisier the Edlebrock is mounted to the chassis than it was just holding it in my hand and testing it. I will try adding a rubber pad between the frame and the mounting bracket I made.