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Unless its too low of an idle speed (sub 800 rpm) or it's an ancient alternator design, it should charge just fine. Modern alternators can put out almost full power at idle.
BTW, your rattle is probably due to the engine running at exactly the right (or wrong) speed and load to cause the engine to vibrate a bit, making other loose things rattle (heat shields). Could also indicate a broken engine mount. At any other RPM or load (like with the lights OFF), it won't do it.
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