Greetings. Have a '96 automatic. For the past few months I've been noticing (as best described as a) whistling sound as I accelerate. Noise can't be heard the faster you go but then obviously, becomes audible when I slow down. Another descriptive example is like a radio frequency sound/pitch. Looked underneath (not a mechanical guy) thinking CV boot issues but all seem intact, no grease leaking, no tears. Vehicle handles well, nothing noticeable that could relate to the noise. A buddy of mine said to check the transmission fluid as the pump could be making the sound but all is well in the fluid department. Any thoughts or suggestions where to look, what it might be? Thanks for reading.

 

Also, I can't replicate the sound while in park and revving engine.

 

I went through something similar, though my 4Runner is an 89, so take this for what it's worth (free internet advice.)

My truck was doing the same thing. It only made the sound while the vehicle was moving, and the sound/pitch never really changed regardless of the speed I was driving. It seemed to be the same whether I was driving 20MPH through my neighborhood, or 60MPH down the highway. Revving the engine while the vehicle wasn't moving would not cause the sound.

Check your u-joints, as that was a common recommendation from what I saw, so I feel compelled to pass that on to you as well. U-joints are part of the drive train, so it seems the sound should be changing as you speed up or down if one of those. Maybe not, but it sure seems like it. Like I mentioned, the sound never changed for me.

In my case, it turned out to be the AC blower motor. The way I determined definitively was by driving and adjusting the climate settings from none to some to full bore. With everything completely off, there was no noise, and with it turned on, even at low settings, I would hear it. I replicated this multiple times. It took awhile to do this, since the sound didn't occur every single time I drove the truck, but only sporadically. I basically had to wait for it to start, then play with the AC/vent settings when it was happening.

I installed a new blower motor, and that completely fixed the issue for me.

After installing the new blower motor, I took the old one apart. It was absolutely filthy so I cleaned it completely and reassembled it to keep as a spare in case I ever need another in a pinch. I suspect simply removing, disassembling and cleaning the old one may have fixed (or at least lessened) the issue, but not really sure.

 

I know this sounds silly, no pun intended, but after I put new side rear view mirrors on my 87 pickup, I got the whistle too. Turns out to be a screw hole on the bottom of the mirror body. I think it holds the mirror in the outer casing or something. Anywho, when I drive, it whistles. Since I use 2/55 AC exclusively in that truck, I had to learn to just tune it out in warm weather.

I know that's what it is, cause I can reach out and put my finger over the hole, and it stops. Take my finger away, it's back. Pretty obvious. I'm not saying it's your problem but maybe something you can check out?
Pat☺