My dobermann is 9. I have two sons, one is nearly 27 and the other 23.  We had only really ever had boys around the house.  4 years ago my eldest son met his girlfriend and 3 years ago my other son met his.  My dog has always been a bit weary of the eldest sons girlfriend, but has been fine with the other sons girlfriend.  The eldest sons girlfriend sometimes makes a noise (she doesn't mean to do it, but it is when she laughs) and my dog runs towards her.  But about a year ago he actually went to jump at her to nip her!  After this when she came over I would put the muzzle on him, but then after a while he seemed okay and she said "its fine don't put it on him".  Anyway, last week he did it again.  My son was sitting next to her and he caught him by the collar as he came towards her.  After this I sat and held his collar - but everytime she spoke, moved or was close to my son just staring at her.  It is really weird.  When she comes to our house, he is fine, and when she is walking about the house, but if she makes this noise it is like he is possessed.  He has known her now for 4 years and is fine around her, goes to her to be stroked, gets all excited when she comes in.  I just worry in case he actually bites her!  I have tried making the high pitched noise and he will come to me, but just looks at me.   

Have you tried in a controlled situation getting her to make the noise but intervening and giving a food reward ?

The more he is subjected to the noise and associates it with a reward he will start to look at that rather than lunging at your sons girlfriend.

I did think of this, but the first time it happened my husband was so angry he just removed him from the situation and put him in the garden - as obviously my sons girlfriend is tiny and my Mason lunging at her is pretty scary thing!  But last week my i grabbed him and my son shouted "no"! But I kept him in the room.  I think she is quite nervous of him, but I know the more he hears the noise the better it will be.  I will try and get her to do this!  I did actually suggest it to my son, and he said "no".  This is why I try to copy the noise, but I don't think I get it right - the pitch must be very high. 
I also have a friend that when she sneezes, she goes on for about 20 sneezes at a time and he is always curious of that, if he is in the garden he will suddenly appear by her side, but he hasn't ever done anything and I must admit, as soon as she sees him coming she stops.  She says he is her instant cure for her sneezing!

Don't tell him off try he reward as it will focus him on something he likes rather than be told off. Especially because of his current reaction.

Telling him off the way he has been already is a seriously bad idea - you're already worried about his reaction to this noise, adding a massively negative association to it as well (punishment, whether verbal, or physical - grabbing collar, shutting out) is only going to make his reaction worse.  Dobes are very sensitive dogs.  The best you could hope for is temporary suppression of his behaviour, which is actually a lot worse - it's basically bottling up the emotion and as with people, hidden emotions have a nasty habit of exploding in a much more dramatic way later on.  He'll become more tense around her and more likely to react.

Far better to reward as has been suggested.  Could you perhaps record her making the sound?  Then you could do some work pairing it with good things when she's not there, then work with her doing it herself so you've got a better chance of success with her present.  Or, have her do it in a setup - same principle, staged work before the real thing, so to speak.

>Could you perhaps record her making the sound?  Then you could do some work pairing it with good things

Ace!
Great idea Nikita.

I must admit when it happens, it is just so quick - I don't think she even realises she makes the noise.  The second time she did it - he didn't react straight away it was a couple of seconds later!  But I will get her to make the noise and I will record it and see what happens then. 
But to be honest if we hadn't of grabbed him I really don't know what he would have done - he is very powerful and our instant reaction was to do this.  What are you saying that we should have not intervened?  We were all taken quite by surprise.  I was so upset, as he is a very loved and adored part of our family.

Oh no, not at all - I would have intervened as well.  Safety is paramount, of course.  But how you do it makes a lot of difference.  Reacting with such anger when he's never done it before is excessive perhaps, but understandable; shock and surprise often results in an angry outburst, and not just in people - I've done it myself.  But subsequently he's been told off for it and that is what you need to avoid now - he'll likely have a huge negative association from that first experience, so it must all be positive from now on.  If he does react remove him CALMLY, no words, no angry body language, just walk him out of the room, but if you can set things up to work on it you should be able to avoid that happening.  :-)