Sloshing sound inside Explorer ST. Have you checked your SPARE Tire well?
I suspected I had a leaky Right Rear Quarter panel window and spoke to the local dealer about it. Service Manager hadn't heard about any such issues, though he had another ST coming in for the same complaint, he'd schedule mine after our upcoming SoDak trip. While on the trip, discovered I had a very annoying high pitch whine (Really, it wasn't me) in that same area, then while driving down some washboard dirt roads in the Badlands, heard a constant creak/squeek/damn annoying sound. Had my wife drive with me in the far back and nothing I put pressure on including pushing pretty hard on the RR1/4 window would stop that squeek. So After cleaning off some of them there crickets (see pic below)
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We found the trail of water in that quarter window leading down to a ...
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.very wet Spare tire well pad
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All drying in the sun now, back to the dealer
There are other posts from a ST Owner whose spare tire well had 2 - 3 inches of water in it.