This has been an ongoing problem for at least a year now and I think it's about time I take care of it in case it is something serious. My car is a manual 2001 V70 T5.

When I let go of my brake pedal, when idling or driving, there is a loud hissing noise coming from near the pedals. If I press the accelerator in all the way, the hissing stops. If I accelerate using cruise control the hissing continues. If I turn the car off, the hissing almost instantly stops. Most concerning is that if I am driving for a while and not touching the brakes, especially on the highway, the brakes tend to get extremely firm for a moment before I can depress them all the way. This could be very dangerous in certain situations where I need to stop quickly.

Any ideas what this might be?

 

your brake booster has a leak. Try resealing the gray bellow boot of the booster at the firewall inside the cabin, to see if that fixes the hiss. If that fails, you need a new brake booster.