Car makes a knocking noise when started

Valve tappets? Do you know where the area of the knocking is coming from? If it only does this when starting cold, you might consider going to a heavier oil. When you have the oil changed, what grade of oil do they put in? In winter, due to the cold up in the Northern areas of the states, it is common to put in 5-W30 weight. In hotter weather is should be 20-W30 or even straight 30 weight if you live in the high heat index of states. I have over 120,000 miles on my Buick and the "low oil pressure" light would come on when it was a cold engine. The mechanic that had done work on on the gasket replacement changed the oil to 20-W50 weight and the problem went away. As engines wear on, the spacing between the bearings and what it is protecting gets larger and the oil pressure drops ever so slightly. It also can be bearings going out in the water pump or pulleys in the belt drive. Try to locate the noise. When you normally hear the sound, raise the hood and have someone start the car to see where the noise is coming from. Sometimes 4 ears are better than two.