Actually Brennan, you're not the only one.  I've never been with a Asian working girl, but have dated a civvie a while back, and I can tell you it was pretty normal for her.  I actually found it annoying at times.  Although, when she had consumed some liquor, the noises subsided...go figure.

The Japanese have very particular ideas for how women should look and act in order to appear attractive to men.  It's definitely a cultural holdover from the past when a woman was always supposed to be submissive.

One of the requirements is for them to speak in an artificially high, almost little girl-like voice.  It's an affectation, but one that becomes so ingrained that it becomes the only way they talk in public.  You hear it everywhere you go in Japan- it is the accepted norm for woman.

The second quirky cultural preference is their well known fascination with school girl porn and school girl sex.  This has been a problem for years, but especially since the age of the beeper, cell phone and internet.  Some high school girls offer sexual services to business men after school and before they go home.  There are hourly "love" hotels that cater to this market.

The money goes to buy the best designer handbags, shoes or clothes.  The pressure to have the latest status symbols, like Gucci or Prada, is so high in some social circles, that 16 year old girls will turn tricks in order to afford things they could never dream of owning.  There are plenty of girls providing and plenty of men interested in their services.  It's a subject that has been written about often in Japan as they try to address the problem of school girl prostitution.

Now getting back to your question,  Japanese porn, therefore, requires that women be both young and very submissive.  They must also make the appropriate little girl sounds that convey both youth and submission.   In real life, Japanese women tend to be more passive in bed than Western women, and this is also reflected in their porn.  

While Japanese men find American women sexy and interesting, and have no problem with them being more sexually assertive or aggressive, the same does not hold true for Japanese women.  It's a novelty to see a blond American woman act in this way, but a Japanese women is expected to act differently in order to be considered feminine and sexually attractive.

In Japan, women that are strong and aggressive are viewed as unfeminine.    So Japanese porn usually has strong undercurrents of women that are being "taken" and forced to do something against their will by a stronger male.  They are often restrained (Japanese bondage is legend), or humilated (they invented Bukkake, and peeing and scat are a staple).  Consistent with that, the women in porn are also examined, poked, prodded, and treated more like sex objects than sex partners.  The little girl sounds reinforce the feeling of youth, sexual submission and helplessness that they are deliberately trying to convey.

Yes, it's a cultural thing.

Some say I sound just like Japanese porn...

I guess some guys like it, it's just natural for me during vigorous play.... And yes with alcohol I become a bit more vocal

I prefer it not to hurt, if that says much...

Omg don't be sorry for asking this question!  I'm asian american and it shocked the $h!t out of me the first time I witnessed it.  It's cultural.  My Japanese buddy says "all gaijin(foreigners) say Japanese women make sex noise like pain.  It's not pain, it's normal."  Uhh... ok.  Then he went on to say "I don't like gaijin women sex noise.  It sounds like they are not enjoying."  So there you go.  Pot, kettle, black.  And no, for me, it's not a big turn on.  I find it disconcerting. Whenever I've been with a "country" girl that made noises like that, I've stopped to ask "Uh... are you ok?"  They just look at me like I'm nuts and say "Why did you stop?  Are YOU ok?"  Haha, no I'm not but we don't have time to go into THAT.