Did you just rebuild one flipper? The manual calls for FL 20-300/28-400 in both places. You can find the manual and schematics at ipdb.org

The coil value has nothing to do with the buzzing and here is why. There are power and hold coils built in each of these coils, hence three lugs instead of two. The power is the "20-300" part. 20 gauge and 300 turns of wire. The hold coil on both are 28 gauge and 400 turns (wraps) of wire. So the power coil on the incorrect one is a bit more powerful than the correct coil, but the hold coil is the same. It is the hold coil that is active when the flipper is held up, so in this case it has nothing to do with the buzz. Swapping it out will give you the correct power value but won't fix the buzz.

I would swap it out anyway, but pay attention to the hardware installation and try to get everything perfectly straight. Make sure the flipper brackets are perfectly aligned. The problem is in the hardware installation/alignment and may or may not be completely fixable.