You should check the transmission fluid with the engine warmed up and running.

The separate differential fluid fill plug is on the rear of the transaxle, just in front of the driver side tie-rod. There is a drain plug below it, but never open this until you're have opened the fill plug...

How high is high RPM? Could it be the exhaust vibrating?

 

I checked the fluid both cold and after about a 25 minute drive, both looked healthy in terms of levels and colour etc.

If i put my foot down i hear it or if I'm gunning it up a steep hill, it will ring then as soon as it changes gear it disappears. I'd say It would be the equivalent to when flooring a manual in 2nd gear, that kind of RPM. I not sure if I'd be qualified to say but i don't think its the exhaust.

 

The transmission fluid should be checked warm, with the engine running, after slowly shifting from Park to L and back to Park. This way the fluid fills all the passageways inside the transmission.

Can you reproduce the sound standing still just revving the engine? Maybe a loose heat shield on the exhaust manifold? Or something else loose?

The drain plug for the differential is on the bottom near where the drive shafts are. The fill plug is facing the firewall. To check or fill the fluid you remove the fill plug and fill until the fluid starts dripping out. The level should be even with the fill hole. It takes Dexron-II (or equivalent) type ATF. Do not put manual transmission gear oil in.

 

If I had to bet it's a broken baffle in the muffler, possibly a heat shield too. My old 93 made a noise like that when the muffler went, I had gotten some snow packed into it the prior winter due to extreme snow fall, it melted and rusted it from the inside out. As mentioned revving the engine in P or Neutral with the parking brake on and foot on the brake will probably confirm this. A differential whine sounds like hydraulics, not really like a ringing in my experience.

 

Whenever I'm at high RPM i hear a high pitched ringing sound that will cut out straight away as the gear shifts or if I ease my foot on the accelerator. Through a bit of research it brought me too checking the Transmission fluid and specifically the transitional fluid. I eventually found that my 1.6 3 speed auto has a separate tank for the transitional fluid but while I find lots of references to it, no clear info on where exactly this is on the underside. Can anyone help, maybe with some very handy photos?? In addition to this, I did decide to check my transmission fluid as I'm not sure if this has ever been changed in the 5 years I've had the car. To my surprise i found it to be very full (even possibly over filled) with the fluid being the pink colour I was hoping to see. So given this, is it even worth checking the transitional fluid? I assume so if the 2 are totally separate and there could be a leak? Alternatively if anyone has any other theories as to the high pitched ringing sound i'd be keen to hear!

My 1999 Chevy Silverado 1500 is doing this exact thing, when the truck hits about 3000 rpm then immediately cuts out as I push the break or it switches gears. Did you figure out what this was?

 

Also if you want to check the color of the transmission fluid, drain just a little into a small container like a shot glass. The fluid my look clear and pink on the dip stick but may be very dark and dirty.