The RV that blew up in Nashville on Friday morning was playing a recording that warned it was going to explode, the city’s police chief said.  The recording said “a potential bomb would detonate within 15 minutes,” Metro Nashville Police Chief John Drake said during an afternoon news conference.  Officers who had responded to reports of shots fired in the area “decided to evacuate the buildings nearby” and were knocking on doors when the RV exploded, Drake said.  No evidence of a Christmas Day shooting was uncovered before the cops came upon the RV, officials have said.