Auto Transmission noise ?

Hello all. I will be taking my truck to the dealer to have them listen to a noise I have that seems to be coming from the transmission. More specifically, when the engine downshifts, when coming up to a stop sign or just slowing down to a near stop, I hear a slight siren like noise when it downshifts into the lower gears. Not a big deal, as most transmissions seem to do that, some mfg's trans seem to do it more than others. However, I also notice the same noise - but a bit louder and more constant) when going slow (around 20mph) on a relatively flat, or slightly descending grade. Kinda sounds like the noise you hear when you go fast in reverse.

I was wondering if anyone else has noticed this in their Tacoma, and if so what might be the cause.

Thanks in advance,

 

There was another thread about this tranny noise, I remember that others heard something like you.

Mine makes a similar sound, but it's not that noticable. I switched my ATF to Amsoil, that seemed to help mine.