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If it's a knocking noise it could be due to two different things. Low grade/bad fuel, if you run fuel that has not sufficent ocatne the engine will start knocking. You shoud try adding a octane booster. The other things is the lifter has gotten clogged, or the oil used is wrong density. You should run a motor flush and add proper oil, a good idea is lucas oil stabilzer. Lucas will retain much more of the oil helping the eninge run smoothly and without noise.
They will find it. This car is under warranty and this is not acceptable. Have them solve the problem. Let me check the lifter, i have a feeling the lifter that the car came with may have too small opening. Have them resolve the issue as i am sure you have pay big money for it.