You need to make a clip, like this:
(Funny that the first upload of an engine problem sounds exactly like a WRX with exhaust mods??)
For me this was play in the crank bearing/spun crank bearings, apparently a common woe for this car/motor.
It started off pretty subtle and got very loud without much driving.
When it was subtle I could keep it away by keeping the engine under load and once the RPMs were up it'd die off.. only to return as I'd let the engine load drop and the RPMs dropped.
I have a video I recorded of it on my camera if needed?