#1
i've been wondering, i can sweep pick rougly, but for some reason I can't mute cleanly the f**** G string. let's say i'm playing this:

e|-12p9--------
B|--------10----
G|------------9-

as soon as I change my fingers and press down on G9 there's a buzzing sound. before i even pick the string it buzzes.

how can i avoid that and mute properly?
#2
it could be normal fret buzz, in which case you might have to adjust your action a bit. If you can't hear it through the amp then I wouldn't worry about it though.

You might also need to either pick the G string faster or press down on the fret a little slower. If your timing is slightly off that could cause some excess sound.
#3
could be making accidental contact with your picking hand. Just go through it slow to make sure your hand doesn't brush the string before your pick.
#4
your picking hand may be touching the string before you pick it, depending on if you're anchoring your hand on the bridge or not. that's the only thing i can think of. it's probably just technique and will eventually go away with practice.
Gear:
Fender CP Jazzmaster
Schecter PT Custom w/ Dimarzio crunch lab/liquifire
Marshall JCM2000 DSL+Orange 4x12
Orange Tiny Terror+Mesa Electra Dyne 2x12
Boss TU-2/NS-2/DD-6
Maxon OD808



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#5
Hey man, here you have a sweep picking tutorial : http://www.guitarlearningtips.org/guitar-technique/sweep-picking-exercises/ that i wrote a couple of weeks a go.

Give it a try, i had a very good feedback from my readers.

PS: This is will work if you have your action in good shape and it has to do with technique.