Think sssssssssssssssssssssibilance and you'll get the idea. I don't know what the actual cause is, but the effect is artificial emphasis of "s" sounds. The normal test for a sound system is to say "sibilance" several times into the microphone, maybe that's where the word came from.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8