New brushes vary in composition from one source to another. Some have a tendency to squeal more than others. What I find helps reduce or eliminate this is sanding in the brushes to match the commutator. Here is my video on the subject.
https://youtu.be/1NiBylwzJ2s?si=qnXDNPjp9zklT0aK
And doing the same job with KR Wilson brush sander.
https://youtu.be/dGLAHBbpK4o?si=gBjK4YF_MLArPjLc