Number one hypothesis here is knock under load. I'm not sure how to track that down but some ideas are to make sure the engine is not running lean, that the knock sensors are connected and mounted, that you are using proper octane fuel.

Did you grease the poly-bushes? If they get dry they will groan. Part of the reason I don't use them. Some like them but I don't as they transfer more vibration than good old rubber. Each to their own preference. If you have changed these items I am now scratching my 2 brain cells to think what else...