Okay that was over 10 minutes so I'm not an expert I admit it.
The backside fungal dust is deposited on the Pressure Pad. When the tape moves, it experiences friction with the tiny dust particles over the pad, making the noise. This noise is somehow picked up by the Head and reproduced as a high pitched squeaking noise throughout the playback
AM/FM Stereo Radio Cassette Player AS-620R Radio CASIO Computer Co., Ltd.; Tokyo, build 1995 ??, 19 pictures, Japan, schematics, tubes, semiconductors
www.radiomuseum.org
But I was right and I have no idea why? Next up is your tape heads I'll just get a basic example off the interweb
I'll be back
Below is a photo of your tape head pick up. If you have that high squeak sound throughout playback of the tape then we're on the right track. Has stated above thorough cleaning of the tape head pick up you should be able to see it immediately when you open the case take a photo I like to take a look.
It is your old tapes that is depositing the fine dust particles after cleaning try a new tape or the newest one you have see what happens
Also if you scroll down you'll see similar threads that may point you in the right direction heck I never noticed them till now.