Actually now that you say that, I only notice it with other passengers in my car and that's the only time I run my air conditioner.. It might have something to do with the air conditioning running I noticed it 3 times on the way to work yesterday (in first or second gear at 1000rpm, using the accelerator to hold a constant speed in peak hour traffic). Two times I had the chance to turn off the A/C which, by chance or not, made the vibration go away! Check that yours is the A/C, which is probably a common trait. If you are not worried about what it is, you probably will stop noticing it! Yes, I can confirm that this definitely happens while the A/C has been running and once I turn off the A/C it will start to go away. Does anyone know the reason behind this, or should I still be talking to the Toyota service centre about it?