I am new vios owner and what i have found out is, under the vios body it have a metal bar to protect things to hit the body. When you go too fast on a bump, your car body will jump low due to the spring and that metal bar will hit the bump. If you are fetching 4-5 people,it will still hit even going slow due to heavy. But don't worry as it looks like that metal is quite hard and it won't damage so easily. ( Last time when i go too fast on a bump and i hear that hitting sound, i went down and check the metal have no problem and somehow the bump on the road has cracked    )

very normal on 3rd gen vios. due to bad design of lower arm and anti roll bar design

Try turn steering left and right when go thru bump.

Should have no sound.

Mine got no such issue with small or sharp high bumps.

But I met this issue when I was in KL trying to exit a Hotel Carpark. The exit was a high slope up with about 110degrees angle at the exit. I had 4 people in the car though.

Actually this is quite expected because some of the places in KL are designed terribly due to limited land spaces

i reported during my 1st service.
They will assign their technician drive with u to hear the noise.
If he confirm the noise is there they need to record the noise to send to HQ for spare part request.
It took almost half day.
After a week they called me when spare part is arrived then set an appointment to change the lower arm.
i sending my car to Wing Hin balakong.
If u near to SC Cheras branch better go there.