What's that noise on cassettes?

I popped an old cassette into the tape deck the other day, after not listening to one for years. When it came to the end, it emitted a high-pitched noise for a few seconds. Flipping it over, the same noise was heard at the beginning.

I’ve heard this noise on a variety of tapes - why is it there?

Some commercial cassettes had a frequency test tone on the tapes. It was about 7 tones. Is that what you’re hearing?

The “glue” that binds the magnetic material to the tape has bled thru. Some people have “baking” remedies but I’ve never seen one work. I consider such tapes a loss.

That must be it, as it steadily changed in pitch. It sounded too “clean” to be the product of degradation.

I think I know this. It’s an acoustic marker put on the master tape from which the cassette copies are made. It tells the machine that duplicates the cassettes when Side 1 ends and Side 2 begins so it can reverse and lay down the second side. This is all done at high speed, of course.

An industrial cassette-tape duplication plant once opened up opposite a place I worked, and I went across to have a chat and see how it was done. The equipment involved was pretty awesome. Anyway, that’s what the guy said that little wibble-wobble tone was for.