Just because you haven't seen it doesn't mean it's impossible. The wheel bearing could be grinding enough to cause the wheel to not glide properly. He pointed out that it happened when he was gently on the gas, but not when he was hard on it, but that when he was off the gas that it was also fine. He pointed out that it only pulls when it makes noise, which effectively rules out an actual alignment issue.

You're hearing a knocking noise...try and get a friend or family member to see if they see anything when the car is rolling and you're hearing the knocking noise (Assuming you can do that safely at low speeds.)