Absolutely bizarre fret buzz issue - help please!

Neck might not have enough relief? A sort of reverse buzz can occur between the string length between the nut and the fret if the neck is too flat.

 

I would try raising the g's saddle a smidge and see if it works itself out, if so and the action still works for you just play it if not check your relief as cxcxcx mention try adding some relief. Have you put a straight edge on the board to see if you have a high fret?

 

I was thinking - if you tie a piece of old, clean t-shirt (Something that won’t damage your bass) around the 5th fret, it will dampen the string length between the nut and the 7th fret. If you try to get the buzz like you have been, and hear no buzz after tying the cloth on, I may be right about not enough relief.

 

Have you tried to restring the g? Loosen and redo? IDK make sure it isnt twised? Check saddle springs that particular freq might be causing it to bugout a little? GL

 

Does it buzz unplugged? It may be something in the room buzzing.

 

It’s a very common form of back buzz - i.e. fret buzz between the fretted position and the nut. If you fret the E string at the 7th fret and pluck it between the nut and fretted position, the buzzy note you will hear is D. When you fret the G string at the 7th fret, the resulting note is D. That note on the G string is causing the back part of the fretted E string to sympathetically vibrate, thus rattling on the frets. Fret the same spot on the E and simultaneously play an open D string for the same effect.
The solution is to add a bit more relief or raise the E string nut slot.