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I'd be curious if you get any smoke when you take the oil cap off at idle. I've seen two cars have problems like this that had broken ring lands. Not saying that's your issue but sometimes the car will seemingly run fine for a while with them broken.
This RPM range is right in the "stumble" area that a lot of people complain about with these cars. It's unfortunately a defect in the fuel pressure regulator design on these cars. To fix it you can ditch the stock one and either put the Cobb kit in or you can get an aftermarket fuel pressure regulator. This seems to help a lot in addressing the fueling in that area which can sometimes also fix the knock in that area.
Just things to check out.
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