I am having an issue with the gandalf/POTD diverter gate. It closes fine when activating war of the ents etc, but doesn't open and will just become slack. Vibrations and pops will cause the gate to drift and block the shot entirely. Any advice on how to get the gate to return to the normal "gandalf orbit"?

Treat this just like any other stuck flipper mech...Return spring, coil sleeve, plunger. Or just get a left flipper rebuild kit and rebuild it all.

Is the diverter gate supposed to vibrate and make noise when its open ?

No.

Yes it can. It's a coil that is energized. But if it's very loud or vibrating a lot then you probably need to adjust it. It's in the manual. Manual is available to download from IPDB and Stern as well. IPDB version is nicer/cleaner. Stern's looks like it's a scan of a copy of a copy of a copy.

So yes it can make noise but shouldn't be as loud of a noise as the tower when it's slumped though.