Brother!
I was having same problem..... it is steering inner rack end metal bushes.... force them to remove complete steering assembly and put new bushes.... steering column bushes and cross will also be inspected along with.
The warranty guy told me at that time " Sir ye har 3rd / 4th Grande mein issue aaya hai.... we have changed so many columns. Aap time nikal kar aein".
Although i sold the car within initiation period but i got it, the problem you are getting....
Low speed when crossing over a CAT eye or small pebble type bumps it gives minor thuds in the steering wheel...
Mark my words 100% and do post your feed back.....
Z links, ball joints, control arms / Tie rod ends sounds diff