My car was humming while at running,put to neutral the humming sound remain..what are the possiblities caused ?

check the PS fluid maybe its low and the steering rack is trying to tell you..
I would let it run an listen carefully under the bonnet for the hum

Take off the timing belt covers if necessary.

A big screwdriver works almost as well as a stethoscope, as does a hollow plastic tube in some cases.

You're lucky it makes a noise while it's just sitting there idling, in contrast to making a noise only while the car is moving.