Lexus LS 2007+ :: Clunk When Braking In The Rain

This problem is recent and only happens when it rains. When braking to a stop, I get a thunk or two. I only having hearing in one ear, so where the sound might be coming from. I ruled out loose things knocking around. The last time I was driving in the ran, I got the thunking only the first one or two times I braked, and then nothing. Other times I have gotten it repeatedly in a single drive. I have never gotten it while braking except when coming to a stop. I don't get it when merely slowing down in traffic or the freeway. I back into my driveway and I am wondering if braking in reverse has any bearing on this problem.

I searched here and on the internet. The problem exists out there as I have described it, but no posts that I can find have identified the problem. The truck has disc fronts and drum rears. I took the front tire off on the drivers side when I re-seated the front wheel bearings. I was going to repack them, but didn't feel like pulling the caliper, so I just re-seated them. I repacked them not that long ago and didn't feel it was worth the trouble at the time. I didn't bother to pull the tire on the passengers side. While I was at it, I greased all the ball joints.

I can't recall exactly when I first started hearing the clunking, if it was before or after the whole U-joint/carrier bearing campaign, but I became acutely aware of it afterwards, perhaps because that's when we got a lot of rain. There is absolutely no driveability issues at all, just the clunk when braking to a stop in the rain.