I had bad guide rod bushings, and the feel of the clunk was similar to what you describe, but the clunk came whenever I hit the breaks, especially when I hit them and then released quickly.

If you can, have someone else repeat the conditions under which you feel the clunk while you stand outside the car. If you see that the wheel is falling back and lurching forward along with the clunk sound, then you know it is likely the guide rod bushings.

I was told by people here and by my mechanic that they had never seen one just let go, but personally, I would check it out to find out what it is, as I think there are other front end components that it could be as well, and some other ones could be more critical.

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