sounds like the clutch is dragging. either the fluid is low, it needs re-adjusted, or the clutch disc itself is delaminating.
or there could be an excess buildup of clutch dust in the flywheel. either of that or the pressure plate may be collapsed or has a broken spring
to prove your clutch is dragging, the reverse gear should grind under the same circumstance.
and if you turn the car off before puttint iti nto 1stor reverse it wont grind
have the clutch adjusted or at least look at the fluid or reservoir
and also, any subaru trans is not designed to be shifted into 1st while moving