My '98 Silverado 5.7L had a leak - intake manifold gaskets were bad... so we took it apart and changed them out and got it put back together. When we went to start it, it tried to turn over but was starting very hard if it started at all. We figured it was a vacuum/compression problem so we took it back apart and put different gaskets on it because we found that it hadn't sealed properly due to a faulty gasket on one side. We got it to start a lot better, but it still starts hard and it clacks / knocks. It's a fast steady clacking sound - it sounds like it's exterior from the block. Someone told us it could be the knock sensor, but that was plugged in. What else would it be?
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