Female sound

Hello, this is my first thread. I have been bugged for years with a faint female voice sound while sometimes lightly accelerating? It doesn't happen all the time. My wife and I chuckle when our 525i e39 makes this sound. It is kind of a short medium-high pitched voice, yet definitely female sounding. I can be accelerating straight or turning left or right and the sound is the same in duration and female.
I have looked under the hood, and under the car for anything dragging --- nothing.
Question:
Has anyone experienced something similar? If so, do you know what might be causing it?
crisslo

Does the pitch change at all as the engine revs higher or as the road speed changes?

Posted reply:
Does the pitch change at all as the engine revs higher or as the road speed changes?

Answer: Thanks for replying to my post.
The sound only appears at low rpm for a very short time (not at higher revs). The pitch doesn't change. It is always the same sound. Also keep in mind, it doesn't always make the sound. I drove over a 100 miles yesterday and another 100 miles today mostly in the rain and cooler around 55 to 62 degree weather. No sound was heard. Come to think about it, I don't ever remember hearing it when driving in the rain.
crisslo

Honestly... I'm stumped. If it doesn't effect your car's performance I wouldn't worry about it.
Anyone else?