Has a chatter when accelerated. Sounds like a pencil stuck into a boxed fan. Comes from center engine. Loses power uphill.
Is the check engine light on? How many miles on this Subie? Does the chatter get faster as the car goes faster? Or when the engine turns faster when y rev it up while in Park?
Is the OP familiar with what spark knock sounds like?
Also called “detonation” or “pinging”, this is usually described as sounding like pebbles being shaken in a tin can, and it typically takes place when climbing hills and when accelerating.
If that is what it sounds like, then I think that your “chatter” is likely to be a case of spark knock.
This can be caused by a variety of problems, including but not limited to…
Spark plugs in need of replacement
An engine that is running too hot
A malfunctioning EGR system
Cylinders that have a lot of carbon build-up
Years ago, ignition timing that was advanced too far was also a major cause of spark knock, but I don’t think that your engine’s timing is adjustable.
Because spark knock can destroy the engine’s valves and can even damage the tops of the pistons, this is something that should be remedied a.s.a.p. I recommend a trip to a qualified mechanic’s shop for diagnosis and repair.
Manual trans? Does using a lower gear help? Automatic trans - does it downshift when you mash the accelerator pedal, as it should?