It's a little difficult to explain without going in the physics of the thing but, ideally, every note should have its own octave hole. On a sax there are only two (usually). So the position of whichever hole is open is going to be good for some notes and not so good for others - it's a compromise.
Nicks post above is right. Could you tell us what make of sax you're having snags with?
The sax generally has 2 octave pads on one the crook and one on the body. the crossover between octave pads happens when playing any note that has the G key down to any not that doesn't.
The crook one should open when playing the notes A -to - C#. All the other notes the key on the crook should be closed and the body octave pad open.
It is sometimes the case that one hears a hissing sound when playing an A because the octave pip is closest to your ears and generally others can hear it.
Good venting can help sometimes. How far open is the octave key on the crook opening when you play A. (do make sure that the other octave pad on the body is fully close when you play A and vice versa when you play G)
The best way to clean the octave holes that i have found is the interdental brushes you can get from chemists and dentists,they come in different sizes and bend to the position you want,the body octave pip is harder to get at then the crook one.
You can also use a pipe cleaner if you wish.
These holes can get gunged up with time and need to be cleaned out regular.
As said above A/G is the crossover between the 2 octave pips.
I'm very late to this discussion, but on my (awful plastic) Vibratosax alto, I get all sorts of weird buzzing sounds on some notes. Mostly, it's due to sympathetic vibrations, possibly with the coil springs it uses under the keys (a truly terrible idea).
In particular, that makes the little rollers on the palm keys and low Eb/C keys dance around with certain notes - resting my little fingers on them, or gungeing them up with cork grease, helps a bit. I still have some more buzzing noises to track down
Definitely a buzz in my case - things rattling about at high speed.
I've just been following your advice about "lost motion" (a term I didn't understand, straight away) and aligning the upper stack keys - in another thread, jbtsax.
The Vibratosax is a marvel of half-assed engineering, IMHO. I bought it from sax.co.uk, so it was allegedly set up by a professional when I bought it, a couple of years ago (I'm not sure what their profession was - that wasn't made clear). Since it's all plastic and coil springs, nothing much has changed since then - it's never been right.
There was enough play between the front high F key and the B key for those to be banging together - the spring of the front F key is probably picking up vibration and banging the front F touch piece against the arm of the B key.
There was also a sizeable gap between the A key and the bis key. When i first got it, the G# touch didn't move very far, because the spring was clogged with too much hot glue - they use that at the factory to secure one end of the spring. That was my first clue that whoever allegedly set it up was completely baffled by it.
One good thing is that the lower stack has adjuster screws for each of the FED keys against the bar that works the auxiliary F key (if that's the correct term), and indirectly lets you do the two fingered Bb (left index, plus any right finger on F, E or D). Also, the pads self level, of course.
Yesterday, I managed to adjust it to play some notes more easily than my Trevor James alto currently does (it needs a service). Not nearly as in tune though. The upper stack was opening too far, especially the G key - so those notes were sharp. The notes from low Eb downwards play so flat that it's hard to get Eb much above a D, in A=440Hz tuning, and low D just seems to have to be played very low to avoid it jumping to the next octave due to lipping it up too much. I settled on "Verdi's A" tuning for it instead - A=432Hz. That's easier to achieve.
I've put some pieces of teflon tape to close the gaps, and stop the A key opening as far - that seems to roughly take care of it, though I haven't played it yet, so I'll probably need to experiment some more. My upstairs neighbour is in, so I have to wait.
I've only just realised today, that there's no link between the low B and C# keys - so if I touch the roller on the C# while playing a low B or Bb, it's going to pop open. There's an arm from the C# key for the B key to press down on, but nothing on the arm of the B key to make the connection. Possibly, they decided to just leave it off, because the key work is not rigid enough for that to work well.