Don't think it's the synchros because all of the gears shift fine, or at least as well as they can considering the shift gates.
It's only when the engine brakes itself due to no throttle being applied. Once I hit the gas or the clutch, it goes away.
I also don't think it's the clutch because I just had it replaced, and it made this noise before and after I replaced it.
Doesn't make this noise when I accelerate, so I don't think it's a valve, only when I engine brake.
I especially notice it when cruising around my parking garage in 1st gear because everything is echoed, and when I go around turns, I let off the gas sometimes, and that's when the noise happens.