Knocking sound when accelerating from a stop or low RPM.

HELP! Been to 3 mechanics. Two that are GM dealers. and I have gotten 3 different answers, from its normal to it needs a high pressure fuel pump. Car runs good. But when it is warm, not cold, I get a mild knocking sound when at the lower RPM range. 1000 to 2000 RPM. I don't hear it while in neutral and revving the engine, but when accelerating in gear from a stop I hear it. I can also get the knocking noise to happen while power braking it. The slower I accelerate, I hear it more. 2012 with 50k miles, Bought it a few weeks ago, went on a trip, got home, then I noticed it. Oil is correct. One GM guy said he hears it but nothing he can do. Said engine is fine. Other GM guy said it needs the high pressure fuel pump??? Its like $500 dollars. Could a HP fuel pump make a knocking noise, while warm only, and still work fine and not leak. I have been bashing my brain for 2 weeks now trying to figure this out. I wend and drop 2 other Traverses, that did not make that noise, so I know its not normal. And I don't think its the noise that injectors make. Any ideas would be helpful. Not a CV axle, does it straight ahead as well. thx.