I had a squealing noise in my Roomba too. which I could trace down to the gearing of the extractor brushes.
The motor was slightly displaced so that the gears didn’t connect properly. I was able to fix this temporarily by adjusting the motor with a piece of plastic. but the gears were worn down already so that I had to replace the extractor housing (including motor and gearing). was not a lot of trouble and is totally doable!
I found the housing in the Roomba shop, but it was a hidden link I only found using google… I am not shure what the situation is where you come from but here in Germany this was also the cheapest offer!