I have a 2000 Chevy Silverado extended cab 1/2 ton longbox. About a week ago, it started sputtering at start-up. There is also a whistle coming from what sounds like the engine, and my husband swears there is a slight knocking sound also coming from the engine, but I don't notice a knocking. The fuel filter was changed less than 3,000 miles ago and we put a bottle of injector cleaner in right after that is supposed to be good for 3,000 miles. Does anyone have any ideas of what could be going on? I'm truck stupid so anything would be helpful. Thanx!