Hard start, noise under vehicle

Vehicle is a 1999 chevy silverado, 2wd, 1500 base model with a 4.8 VL engine. Mileage is 211,500. Oil change every 3 months. Fuel filter, gas pump, cabin filters are all brand new. The trouble I am having is when I turn the key over it takes longer for it to start. Before it would start right over, and now sometimes the lights dim just a little and it is fine. I had the alternator, Battery, something else tested, don't remember, but I know it checked out fine. The vehicle is making a noise under neath the front of the vehicle. I don't know what it is, the noise seems to exist when I press the gas, other than that if I press the gas and then let go it doesn't make the noise. Had got a word that it might be my vapor pressure sensor not giving enough gas for the vehicle to start. : I do would like this resolved so I have some idea where to begin, because I do need my vehicle to commute to work each day. Thanks for your time.