Check Gauge Light comes on when tachometer reaches between 2000 and 2200 RPMs. I also noticed a rattling sound when the AC is on.
Based on your question I believe that you are having an issue with the serpentine belt tensioner on your truck. The serpentine belt is on the front of the engine and drives all the accessories attached to the engine. This includes the a/c compressor, water pump, alternator and power steering pump. It is possible that your belt tensioner has failed. This will cause a rattle sound under certain conditions and could cause the belt to slip which could stop the alternator from charging the battery. If this occurs, this will make the check gauges light to come on and alert the driver that there is a problem. I would keep an eye on your battery charge indicator and water temp gauge to avoid any issues until you can ask for help from a mechanic.
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