Thanks for the suggestion cc260. I'll make sure to do that when I replace the strut mounts, but it seems unlikely because the springs are not moving when the wheel is tested when the car is up on jacks.

20,000 miles seems really soon for Bilstein HD's to fail, but the noise does seem to be coming from inside the strut, or hopefully the strut mount.

I'll update this post when the mounts come in the mail.

Dan