Trying to help with my freinds 1992 registered Honda Civic CRX VTech 1.6

When driving there is an intermittent noise, becoming more frequent, from the engine compartment.
It sounds like an air compressor and the speed is constant, not related to the engine or road speed.

Just wondering if anyone could cast some light on this as I do not know what can be causing the noise and I am not conversant with this model.

Thank you in advance for your time.