On the subject of the 12V Murata reg possibly not getting enough head room; I've swapped the Caiman's reservoir cap for a 35V film version, removed the input diode, removed the resistor behind the 5V reg and set my linear supply's output voltage to 16V with the load attached. So I know that both regs are getting 16V, which is well above their drop-out voltage. Despite all that I still get hiss and whistling. So we can safely rule out the theory of the Murata regs not getting enough voltage.