uhh... is my 904 "chirping" under hard upshifts???

A few years back, I had my 904 rebuilt to handle more power from a built small block as I prepared for the new motor. My swap from the 318 to 5.9L Magnum was a big jump but I foresee more power coming down the road and wanted to build it accordingly.

So Best Transmission in Springfield, MA has been around since the 60s and the owner has a good reputation for muscle car and drag racing transmissions...I don't have the complete list of upgrades that were installed but upgraded bands and clutches along with a ~5800 RPM governor weight and a TF2 shift kit were part of the build. I am also running a TCI 2600 RPM converter and a deeper pan. Best Transmission admitted that they had built very few Chrysler units but they were confident that they could rebuild it the way I wanted it. I opted to not go for a manual shift valve body, mostly because of the column shift...

So I have only put a few hundred miles on the car since getting it back on the road last year and aside from not having the kickdown linkage tuned yet, the transmission seems to perform fine under normal driving.

Because I still have the fragile 7.25" rear (with the 2.76 ratio) I haven't hammered on it at all....but on the few occasions where I would wind it up a bit (usually shifting manually around 5k) resulted in a chirp specifically not coming from the tires, that I THOUGHT was a belt slippage on upshifts...but after checking the tension on the (brand new) belts...this theory started to fall apart. Honestly, it doesn't make sense that this scenario would cause the belts to chrip anyway since an upshift would slow down the alternator and fan...not speed them up.

So I'm starting to worry that the noise is coming from the 904 slushbox itself.

I'm MUCH more familiar with manual transmissions and nearly every other car I've ever owned has had three pedals...so please excuse my ignorance.

Is this plausible? What would it mean for the transmission?
Yes...the fluid level is good... What else should I check?

Thanks a million for any thoughts!