Q: Engine whines, knocks, and has slow acceleration

In my weekly round trips to work and back, I drive about 120 miles each week. During those drives my car works very well. The problem is that when I turn on the car and look under the hood, while the engine is idling, I can hear the engine repeatedly whining on and off. The sound goes high/low over and over. It sounds like it does when someone is pressing the accelerator, releasing it, pressing it, over and over. There is also a knocking noise that also comes and goes. I can only hear these when the hood is open. When I’m driving, I can not hear these things and the car drives and performs very well. Naturally I am hoping it is a minor problem, but I want to make sure to take care of it now if it’s more serious. My car has 263,000 miles. Every 3,000 - 4,000 miles I change the oil and my handbook says to change every 7,000. Another problem with my car is that it is not accelerating as quickly as it used to. There is no hard hit on the accelerator pedal (sport mode). It likes a gradual acceleration and it does go fast speeds. I replaced the spark plugs, but that did not make any difference. What do you advise that I can do to fix my car?