Engine Performance problem
1999 GMC Sierra 6 cyl Two Wheel Drive Automatic 250000 miles
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The engine runs rough at low rpm say up to around 2000. Then it smooths out somewhat. The fuel pump makes a higher pitched whine than the usual sound. This problem started off less noticeable and intermittent and without me noticing the fuel pump noise. What should I do to fix this problem?

I would check for codes to start to see if there are any clues there. IF none, I would check fuel pressure to monitor what it does as the problems are happening to see if it is falling out of spec. The were fuel pump connector issues on that era of GM trucks, but I think at your mileage, this would have been addressed.