DUDE i know exactly what you are talking about..
I had the same problem and it was embarassing because you're driving a nice lexus but when you go over a small bump or hit a crack in the road it make this loud click noise coming from your rear brake!!
My rear right brake did that too!
But when I had my new slotted/drilled rotors and brake pads put on, the noise went away.
The noise isn't rotor related, but I think it is somethng related to the caliper/brake pad.
I was seriously shopping around for a new caliper and was just going to replace the whole thing because that's how annoying the noise was to me.
I think the mechanics fixed it on accident when he removed the caliper to put the new pads on.
My suggestion would be to just get new pads put on the rear, what's it gonna cost like only $50 or so? Or just take off your pads and put them back on. I never found out what the noise was but it was gone after I had that work done like I said.
I really got tired of hearing that dam clickign noise every time I drive over a crack or hit a small bump in the road. And if you are like me and think it will just go away over time good luck.. The noise got louder and more sensitive over time, eventually it even clicked when I was just driving on smooth road!! and when i step on the brake it would make a loud "pop" sounding click. There was no problem braking or anything, just the noise was soo embarassing... If you live in the country and no one is gonna hear the noise except trees and cows then whatever maybe you don't want care to worry about it. But I live in philly and every time I drive there are people walking around everywhere, don't want my nice looking car to sound like a clunker....