My daughter who is 9 months makes that awful scream too - I know exactly what you mean - it actually hurts my ears sometimes! I think it is boredom/frustration thing combined with trying out her voice.
She does it in the supermarket when I stop pushing the pushchair and browse the celebrity mags! She has been doing it for at least 3 months now so I have built up a better idea of why she is doing it and react depending on where she is when she starts off. So,
I move her out of the bumbo or give her a new toy/amuse her for a bit if it is because she is bored or feels trapped in her bumbo.
Give her something to nibble on (like your rice cake idea) if she is in her highchair waiting for her food to heat up/cool down.
Sometimes she does it because she is getting tired so it acts as a reminder that she is prob ready for a nap or it is time for dinner and evening wind down.
However, sometimes she just does it because she is excited and wanting to join in with what is going on around her. My husband is a bit "don't react because she will do it more" but I'm not convinced - I think it works better to respond to what is causing the screeching. I am not an over indulgent mother and if she is doing it in shops etc then I just ignore it. However, most of the time it is because she is bored and part of my job is to keep her amused so I will try and distract her and she normally stops.
Hope this helps!