What would cause a buck-boost regulator to buzz loudly and generate high voltage ripple during certain input voltages?
Looks like everyone who said it was the loop compensation was right! But I've led everyone on a bit of a wild goose chase - the problem was a mistake on the schematic that must have occurred when it was being modified from one iteration to the next: R6 is meant to connect to COMP, but is connected to FB! So, it was indeed the loop compensation not working properly!