Hey Gkar (btw we use our real names here),
Welcome to the forum.
If the tape sounded fine before the section that you found to be shrill and distorted, it could be a sticky tape. I don't believe I've had any problems with true "sticky shed syndrome" with BC tapes but I do have a couple that squeal. You should play the tape again and when it starts sounding shrill, turn the sound down and get close to the machine and listen to that section. It may be the tape squealing as it moves through the tape path. If so, you'll be able to hear it. That noise is transmitted to the tape and right on out through your speakers.
Like I said, if that's the case it may be something that was spilled onto the tape.
If that isn't the problem, it could be a damaged tape with any number of problems from poor handling or storage from a previous owner.
If you look at the "Beginners Guide" located above the General forum in a sticky, it goes into several of the problems that may occur with old tapes. BC's are generally in pretty good shape since most of the original owners took good care of them, but as they say,.. s**t happens, and even the most careful owner can have a coffee disaster.
I have a couple of tapes that have unknown problems. If they sound great otherwise, I hold onto them in hopes that someday I'll be able to solve the problem.
If it seems like the tape is hopeless, and the seller has a good return policy, you might want to see if you can get your money back. There are quite a few copies of that one out there.