Subaru Problems? Ask a Mechanic for Answers ASAP
Is the check engine light on or flashing?
It sounds like you might have a misfire happening. This will sometimes sound like a tick or snapping sound under the hood when the coil or plug wire has spark arcing out of it. One method for testing this that works pretty well is to spray water on the ignition coil and spark plug wires with the engine running. If it begins to skip or you hear the snapping it's a good bet you will see the spark arcing that is causing the issue and be able to replace the failed part.
This is where I would start with this since the check engine light isn't on now.
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Thought I would check and see if you needed any more assistance with this?