This morning it was about 75 degrees Celsius and after a slight warmup I started on my morning commute. I suddenly noticed when my car was accelawrating under light load I had my car make a deep grumble upon light load accelerating. It almost sounded like the car was in the higher rpm band. Sounded almost like engine braking. It did it for about 10 seconds and stopped making the sound. It was almost like a noise cancellation issue with it not able to block out the sound. After the 10 seconds it went away completely and was as if the car canceled the noise some how. Any ideas what this is? 2009 Lexus LS460 base model.