Clank sound and won’t turn over to start

My daughter drove the 2011 Expedition to the
store, and when coming out of the parking lot, it
made a noise (loud pop) and stopped.  We had it
towed to her school apartment and had a mechanic
go out to work on it.  The battery tested bad, so we
got a new one.  He took off the starter and it was
bad, so we got a new one.  It continues to do the
same thing. The vehicle had not shown any
problems prior to that day. It had the oil change
regularly. It has about 140,000 miles.

When we turn the key, it makes a “clank clank”
sound and that is it.  Any ideas of what else it could
be?  The mechanic that took a look at it, hasn’t
been able to go back and we can’t find any other
that can go out to see it. The mechanic was
puzzled bc there isn’t a reason why it would have
seized, so he thinks there’s something else but is
puzzled. It will cost $300 just to get it towed to our
mechanic. Any ideas to look at will be greatly
appreciated.