Hyundai Repair: Transmission whining noise, hyundai elantra, diagnostic procedure

Question
Hi, I have a Hyundai Elantra 2000. I hear a whining noise coming out of the transmission in low speeds and specialy
when coasting (slowing down). I am sure it is transmission as when I shift to neutral in motion, the noise goes away.
I just changed the transmission fluid as it had turned color to brown. Any idea if it is a critical problem? What should be done? A mechanic told me that it is from transmission and it is normal and he has heard this in many cars. The noise sounds like a belt noise but I think it is from transmission.
Thank you

Answer
Based on your diagnostic procedure, I agree that it's coming from the transmission.  Most front drive transmissions make at least a little whining noise.  Some transmissions are louder than others, and some vehicles insulate the noise better than others.

Considering that you've already had a mechanic look at it, and he thinks it's normal, I doubt it's an indication that you have a problem with the transmission.