I'm fairly new as well, but have read quite a few threads talking about aftermarket pads. My LS400 does the same thing. It is quite annoying and I find myself minimizing how much I brake when I back out anywhere by shifting it into neutral and coasting out of the parking spot. I don't want people to hear that sound coming from my car! I am certain that my car has aftermarket brake pads, but I just haven't wanted to tear into yet. It can be a fairly loud clunk. To me, it sounds like the same thing as what I have going on.
Thx for the quick feedback. For what it's worth, on mine, the Lexus dealer replaced the front pads as part of the certified pre-owned reconditioning process right before I bought it. I didn't notice this sound for about the first 1,000 miles, then it has been getting progressively louder over the last 4,000 miles. So, your explanation about aftermarket pads would make sense in my situation (the dealer service print-out indicates the pads cost only $50, so you're probably right). Maybe higher quality pads will resolve the issue when it comes time to replace the existing ones. Thanks again!!