If you hear water constantly running or the overflow tube has higher than normal water levels or is overflowing itself, the problem is likely with your fill valve.
If your float submerged, there is water inside the float, or the float is cracked, you will need to replace the float and float rod. If the float rod alone is severely corroded, broken or there are threads missing, you will also need to replace the float and float rod.
Click here to find out how.
If the above actions do not help, you should replace your fill valve altogether. Be gone old fill valve, be gone.