How can I model a circuit for this noise coupling phenomenon
You have electric fields causing displacement currents, as moving Positive charges in one location cause moving Negative  charges elsewhere.

Nested pieces of metal have nested, or series capacitors, and charge is transferred. Your circuit is inside a shield but visible to the shield.

The transducer circuit is a fine target for some of the electric field, thus charges are forced to move in the transducer, until an equilibrium is reached when the charges stop moving. But you've added a *PATH** to the outside world ---- the transducer's output signal and the return wiring. These displacement currents now have YET ANOTHER path back to that mechanical structure.

By wiring the transducer shield to Ground, the shield becomes much more quiet (not totally quiet, as V = I * Z will predict), and thus very little charge is transferred into your circuit.

So you have something like this







simulate this circuit &ndash; Schematic created using CircuitLab

Notice at high frequencies that wire (inductor) to ground will not be close to a short (as it is at 50Hz); the voltage division action (against the 1 ohm Rsource) becomes less useful at high frequencies; in such cases you need optical isolation.