How do I measure very low frequency noise?
A Windows PC can do this for free with Audacity. You still need the coax cables, Faraday shield box if necessary and an AC coupled 50 ohm load. View into a scope and amplify as needed from 0.1 V to 1 Vpp 

Then measure for 1 minute for a smooth response as shown below.

Then feed the noise into your audio input and calibrate with a sine wave.



Some PC's have a DC-coupled mic input, with low offset and others have some HPF. You would choose a 1 Hz HPF.