OpeNoise is one… it will need calibration but if you weren’t concerned about actual dB readings you could use it uncalibrated

It says that the program depends on a non-free network service. Does that mean there is a subscription or other type of payment involved?

Any program that communicates with USB connected hardware via a non-standard, Windows only USB driver as provided by the hardware vendor. In other words, you can’t just plug a device into any OS and start programing it via a standard USB driver.