My guess is that the odds are about 1 in 20 or thereabouts! Think of the surface area touching ground, vs diameter of tire.
Anyway, it's possible in the rain you missed it...you really need someone looking while you roll car slowly over them!
One thing I do after driving through gravel is what racers do to clean the marbles off tires in a race...hard swerving back and forth. BUT, if you do have a nail or screw, that could dig it in further.
Other possibility...just happened to me...is a stone in the dust shield, but that wouldn't be in a particular cycle...just random rattling.
Anyway, many years of experience with such a thumping here...and hope I'm right...if I'm wrong, then we're down to some mechanical things like a half shaft...probably not bearings, but those are 'easy' to test for...but don't even think about that stuff yet.
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A recent set: https://youtu.be/995gLmVEVZk
Mangofy your E46!