I've been noticing an intermittent/random knocking sound from the engine when at fast food drive throughs where the sound can reflect back. Transmission in dive with foot on the brake pedal. If I put it in neutral, the knock will go away. It is in no way constant and comes and goes pretty randomly. I tried putting the parking brake on and getting out but could not duplicate it. Truck runs and drives fine, oil looks good after 3000 miles. I just received a Blackstone kit and will be sending a sample out soon. Any thoughts?