Stern flipper buzz is normal. It is due to how they do the holding power.

Here is a link with a few things to try to lessen the buzz

https://pinside.com/pinball/forum/topic/flipper-makes-buzzing-sound-when-i-hold-it

It has probably always been there you just couldn't hear it over the music and it has gotten louder as the parts have worn. Mine did it right out of the box, made a few adjustments to the coil and switch and helped a bit.

Make sure the coil is tight in the bracket, also as LTG posted in the link file the plunger smooth if needed

The plunger is the metal rod that travels through the center of the coil, as it bangs against the stop when the coil is energized it starts to turn to a mushroom shape.

When you put it back together you want the bracket to hold the coil snug so the coil doesn't move back an forth

Also, in my case I could never get it to completely go away just got it to be less noticeable.

This was the same result for lots of people from what I saw when I searched about it