Sorry, I was not clear enough. From "spectre -h noise" :
The noise is computed at the output of the circuit. The output is specified with either a pair of
nodes or a probe component. To specify the output of a circuit with a probe, specify it with the
`oprobe' parameter. If the output is voltage (or potential), choose a `resistor' or a `port' as the
output probe. If the output is current (or flow), choose a `vsource' or `iprobe' as the output probe.
So you need to probe either a voltage source or an iprobe component - it will then measure the current flowing through that source or probe (the voltage source would probably be a zero-volt source).