Hello, I have a motor I just recently got and got running again. A 69 351w. 10.5 compression. I finally got it running but I have a knock in it. At idle there is no knock. If I hold it at a higher rpm I hear nothing for a couple seconds then I start hearing what sounds like a rod knock. My understanding is you always hear a rod knock it doesn’t go away. But this one come on like I said. And disappears at idle. Any clue?