OK, guys i have a prob. My 3rd G string sounds very strange. I changed it a few times. I re-setup my guitar. What else may be the problem. Btw there arent any rattles. ( Guitar: Fender Highway One Stratocaster ). The sound is like the string is out of tune, or the string is loose.

Maybe its a problem with the nut. Too low and the open G string will take on funny harmonics or buzz as it bounces off the first fret. Too high and it will sound sharp at the 1st-3rd frets.
Or a problem at the saddle. If a groove has been worn into it, it will sound funny on everey note along the string.
you can either:-
A, smooth it over with a bit of emery board.
B, swap it round with the low E string saddle. Not a permanent fix but will get you out of trouble for a couple of days until you can get some new saddles.

If it happens on all strings its likely pickup height or a neck relief issue.

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No no and no

Welcome!

Gently put a little downward finger pressure on the G string right behind the nut (where the G string comes out of the nut, heading for the tuner) and then pluck the G string. If all or most of the strange noises disappear, you then know the nut slot is the culprit where the trailing off angle from the nut cut to the tuner isn't steep enough. Usually, this is the problem.

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You dig?

I think I know what you mean.... Is it a strange 'warbling' tone that gets worse the furhter up the neck you fret the note, so worst of all at the 21st fret? I've had that on 2 strats, at first I thought it was the PUs being to close, I lowered them and it wasn't! It can't be the nut in my case because freted notes sound worse. I ended up thinking it was a genral 'strat thing'. I played one strat in a shop once that had the same isuuse on the B string, not the G.

I'd be interested to know what it is too!

My best advice would be just take it to the nearest repair shop, as i can just about assure you it is nothing major at all, so if you cant figure out the problem yourself get it fixed as soon as possible.