What era machine?

Start by checking the end of stroke switch wiring and adjustment if so equipped.

Usually vibrating flippers are caused by the eos switch not being gapped properly. I'd check those first.

This is an older EM. No diode should be present, as you figured out. Did you replace with the same flipper coils? (These should be WMS EM coils--not all flipper coils are the same!) Are both coils vibrating? If so, might be wrong flipper coils.

If the coils are correct, it's also important to clean the EOS switches thoroughly, as well as the flipper button contacts. Best way is to put a new dollar bill or rough business card between the contacts (folded into a narrow strip as needed), pinch the contacts together on the bill/card, and pull it through the contacts. Then make sure these contacts sit flat against each other with a bit of tension. EOS switch should open just before the top of stroke (say, when the flipper tip is about 1/2" from fully up). The full power-stroke current to the flipper coil passes thru both the flipper button switch and EOS switch in these games.

Faces is pretty cool game; I bet the playfield and backglass are almost perfect, right?