I had installed a pair of new struts onto my 04, about a year later the truck got wrecked and I pulled the struts off that truck for re-use in my 'new to me' 05 Trailblazer. Last week I was about to install those struts ( actually had them both in place - but not bolted in yet ) and I heard a metal-to-metal clang noise while I was about to tighten the pass side strut. Under the strut boot there was a metal "washer" that was loose. It was supposed to be in a fixed position under / at the top of the strut boot, but somehow; it had come loose. Turn the strut upside down and it moved on the strut post to the top of the boot ,.... turn the strut back to normal and it would fall till it hit the body of the strut. I never would have looked for this as a problem if the strut was installed - but I sure as heck would have heard that thing clanging around in there while driving - even over the mildest bumps.
I doubt this is your issue, but the point is, since you've already had the most obvious possible causes check out,..... you gotta' start looking at stuff that's not a "normal" occurrence/failure. It's just not that often that you will hear suspension type sounding problems when driving in a straight line.