If its orange light and only comes when you loose traction on wet ground or loose gravel, then its pretty normal.
if its redlight and comes on highway or while normal driving (without slipping) then get it checked, it could be serious.
Sorry Bipin, you have to replace the belt only. Seat belt have pretension-er in it and if it gets broken completely this what happen and in this part of world people don't repair much things like in India.
Dont drive bro, it could be something serious in engine (if rattling comes from engine).
If rattling coming under the car the something is getting rubbed below like some heat shield or other part, lift the car up and inspect with educated mechanic.