Do you get a similar issue with top E, F and F# ? Besides the observations previously indicated I have found that 3 other things affect the top notes and can lead to the resonance you are experiencing. The most obvious, if it affects all notes G and above is the tone holes, particularly the tone hole in the saxophone neck. I make habit of with a pipe cleaner clearing both tone holes. The one most affecting you is is the hole which the octave key covers on the neck. The other one you can see if you look down the body of the sax with the neck off, you will see a vent pipe near the top of the body.
Also check is your reed positioning, make sure that that the tip of the reed is aligned with the curvature of the mouthpiece tip and that if you press the reed (gently) with your thumb onto the mouthpiece the tip of the reed just reaches the tip of the mouthpiece.
When these are correct you should then ensure that the mouthpiece is in the correct position on the neck to give correct tuning. i.e. following jbtsax advice place mouthpiece onto sax and tune as normal (try and blow naturally, it is tempting to try and play in tune!)
Regards Rikki