I noticed that when my car is in neutral and I rev it I hear the a noise coming from the drivetrain. It sound like the gears meshing togheter.

I does not sound like the through out bearing did in my 2001 Prelude. During idle it does not sound but when you rev the car is make that sound like from the gears meshing. It only happens only from 1000 rpms to 1500 rpms. When it is cold it does the noise more, warm somethimes does it more than other times.

If press the clutch before rev it up the noise in completly gone, it does not sound like a bearing it sound like the teeth of the gears. While the car is runnig there is no noise or weird behavior.....

when you push your clutch in and rev it up do you still hear the noise??
Noting is moving in your trans when your car is in neutral, or when the clutch is pushed in. It wouldnt be gears grinding. Its definatley something before your trasnsmission.
Did you upgrade your clutch?
The only thing i can think of of if the noise is coming from that area would have to be a bearing. It definatley sound exaclty like a throwout bearing

I understand that the gears are spinning when the car is in neutral with the clutch out (not pushed). Once you push the clutch in the drivetrain stop to let the gear engage.

Correct me if I am wrong

Ohhh If I push the clutch and rev it the noise is gone....

In idle (clutch out) there is no noise

Reving it up in neutral (clutch out) noise (kind of teeth meshing, NOT grinding) as soon you push the gas (from 750 rpms), then is gone. I only duplicate the noise exactly when pushing the gas from 750 rpms then is gone.

If I repeat the mentioned scenario with the clutch in there is no noise at all.

haha dam man, never heard of it.
Anything done to your car??? Maybe an exhaust, cobb ap, boost controller? Im not the smartest in the world but it wouldnt make any sense why it would be doing this unless that car wans't stock
The only other thing i can think of is your exhaust, kinda like your honda.
Most of them allways have that dam weird rattling noise in a certain rpm range.
But an 07 sti isnt a honda.
Other then that idk. If you havent voided you warranty yet, id suggest you take it to your dealership.
good luck