Empty your pack, fill your bathtub add a little detergent and submerge your pack. Agitate with your hands and you’ll see all kinds of dirt, grime and gunk come loose. I’ve had that squeaky noise problem a few times. Washing it helps.
Dust and grit lead to squeaks and noises. wash it.
Some times the material of the shirt you're wearing could cause rustling noises on the straps.
Even new, expensive packs can make or have noises when hiking.