You are talking about two different kinds of knock.
The knock that the knock sensor detects is detonation, which if sustained will lead to the second type of knock which is where the engine rod bearings get damaged and you get rod knock and copper filings in the oil plus a huge repair bill.
If your knock sensor is faulty, your engine might also knock as a result since the Subaru ECU uses the knock sensor to safely adjust the timing to whatever fuel you happen to be using (within limits).
Reset the ECU and see if the knock sensor code re-appears, if it does, then replace the knock sensor.
Knock (the detonation kind) can only be heard on load and on my engine at least, I've only ever noticed it when on boost when it can be hard to hear if you've got a loud exhaust. Sounds like small stones hitting a sheet of metal.