whats causing a hummming noise

It could be an intake vacuum leak, an noise that is a resonance related to engine RPM and harmonics or many other things but unfortunately without hearing the noise first hand or being able to do some basic diagnostic tests or be able to drive the car, its hard to say what the source of the noise may be.

check the PCV vale, they make a buzzing noise sometimes

There are many possibilities. A loose part that vibrates under the car.