2000 Ford Mustang Grinding Noise While Idling

About 3 months ago, black smoke came through my AC vents (AC on) I haven't used it since... no other problems until today. Car suddenly has EXTREMELY loud grinding noise while idling and driving slow. Any ideas?? Are these two things related??

one check for youre dip stick for enough oil two have somebody turn on the car car while you look under the car to see where the noise is coming from,pay close attension to the parts close to the cadilac converter.Most of the time its just something loose that shakes when the car is idling,and people mistake it for something bigger,there i hope this helped.