Its a b15 manual tranny. From a 2000 - 2001 sentra se special edition with lsd. It sounds just like a rod knocking. Its got me all freaked out. The car is there idling and u can hear it but when i got under the car it was just coming straight from the bottom of the tranny. If u get next to the driver side wheel (where the tranny is) u can hear it but if u put your ear next to passenger side wheel u wont hear it.