Low tone engine noise with clutch out, becomes high pitched with clutch in. Thoughts?

I've got an unknown issue with my 07 Accent (1.6L):

There is a low tone noise coming from near the upper area of the engine. It sounds like air or fluid travelling through a hose. The tone of the noise has recently become progressively louder, so that it is noticeable over normal road noise when driving. The noise does not fluctuate in tone or intensity with changes in RPM. However, the tone changes to a higher pitched frequency, almost like a hiss, when the clutch is fully engaged and held to the floor, both when in gear and when not in gear.

Any ideas on what the cause may be? And potentially required maintenance?

Thanks,

-TG