Dang. I can hear it in my head now, but I can't put it into words.
Maybe words like "gong, crash, Squeak, trill" in scratchy font to convey the messiness of it? Since it's musical in nature you can add scratchy music notes running along with the sound fx. If you actually show panels of the orchestra leading up, it might help establish those sounds in the reader's mind. Another thought might be to start with small FX and make them bigger and bigger until it reaches the end.
This is a tricky one, but good luck!
I'm interested to see others' take on this and your final solution!