1968 Cadillac Fleetwood V8 66,000 miles

hey, i replaced my thermostat and it went smooth. we had to remove the belt connected to the crankshaft cause it was in the way then put it back in place. now when i turn off the engine, it wont shut off right away. it takes about five secs and slowly dies down then makes a strange loud noies. what do u think the problem is? and how can i fix it? thanks