Why are high impedance circuits more sensitive to noise?
You can model a capacitive coupling between a noise source and your circuit with 3 elements :


a voltage source (the noise source)
a capacitor (the capacitive    coupling)
a resistor (the input impedance of your circuit)


If the resistor has a small value, you won't get much voltage at the input of your circuit
If the resistor has a big value (high impedance), the voltage will be much higher.