That humming noise is the ring & pinion gone bad. Its happen to my first VLSD. The easiest thing for you to do is find another S13 VLSD since it will fit perfectly. But you can also get the J30 VLSD or the Z32 VLSD. But with these VLSD's you will need to change the back blate so they will bolt up to your S13. Also you will need diffrent axles unless you get the 95 and up J30 VLSD. But if you use the Z32 or 94 and down J30 VLSD all you need are the J30 axles and everything will fit perfectly.