Hi, I play a few other instruments and I'm trying to play the alto sax. When I play the octave D, sometimes I get a high squeak instead of a note. And sometimes when I play the octave G, it kinda sounds like a burp instead of a note. Anyone know what might be causing this?

First of all, great choice with the alto sax (see "Battle of the Saxes")!!!

Ok, are you just starting?? If so, it's natural that the D won't come out; it happens to everyone. You just need to adjust your embature (sp?) and keep practicing. The same thing goes with the G, though I admit that didn't happen to me, so I'm not 100% sure of what to do there.

Hope this helps!!

good for you, sax.
anyhow that never happended to me but its common. just keep trying.

What you are most likely getting with your G and D is the upper partial. This usually happens with G on Tenor and D on alto. The reason that this happens is because both of the these notes are the breaking point of your octave keys. Better explained is that between is A and G when your octave key shifts, from the upper octave to the lower octave, finger octave A and then G, you can watch your octave keys switch. This can happen at the D key because you are at the end of the range of where you need the octave key. Unfortunately this is an inherent problem with the saxophone that can be, with practice, overcome.
Make sure that your embouchoure is correct, check out "The Art of Playing Saxophone", by Larry Teal for an excellent reference on this subject.
Second use warm air when you play, almost like you are trying to see your breath during winter.

This may not be the case at all you may have a weak reed if it continues to squeak, or a problem with the horn.

Also Yamaha does a great deal in its collaboration with Eugene Rousseau, to remedy this problem, on their newer horns.

Good Luck
CJ