2008 Chevrolet Silverado Engine knocking

My truck recently started making a knocking sound. When I was leaving work I was accelerating and I got a low oil pressure warning that popped up for about 3 seconds and then went away. The morning after on my way to work I heard a slight knocking sound that seemed to be coming from the engine. But on my way home I never heard it again and it seemed to be ok for a few days. Then just last night on my way out I heard the knocking sound again and on my way home, again while accelerating, I got the low pressure warning again. It did the same thing, popped up for about 3 seconds then went away. I do usually start my truck up a little early to let it warm up, but both times I got the low oil pressure warning I was driving from a cold start. Any idea what it could be?