banging sound rear underneath of the car

What you are describing could be any number of things. It sounds like something may be banging against the body or frame, which transfers audible sound to the interior of the car. It could be anything...bad motor mount, loose exhaust pipe, rough running engine, transmission mount, etc.

You will need safely jack the car up, or put it on a lift, to inspect the components under the car. It could be as simple as a heat shield touching an exhaust pipe after being bent from road debris.

Don't rule out motor mounts by sound alone. A weak motor mount can allow the engine, when revved, to twist enough that something touches causing noise.

A common problem with the i20 is a knocking noise at the rear which also disappears with increased speed. Many forums with no answer. I can help many owners. I replaced shocks at about 150,000 km with Monroes as I had a slight knock from rear. Knock got worse. No noise if shock taken out and replaced. After a few kms noise returned. Hyundai could not resolve. After 17 months I was desperate. Replaced with new Monroes. Still the same. Wide awake Speedys guy reported similar problem with other vehicles. Replaced with KYB shocks (genuine product too expensive). Noise gone. Like new car. Bottom bracket on shock on axle on other shocks had SIDE PLAY and were knocking against side of bolt. KYB's have sleeve inside bolt mounting which prevents horizontal play.

Hi there I have a brand new 2018 Hyundai Tucson that he's doing exactly the same thing with only 515 miles on it. It happens mainly when I hit bumps in the road. After further investigation we found out that the tools that are used to change the tire that are placed inside the spare were not secured and so whenever you hit a bump they bang on the bottom of the car.

You may want to look to see if your tools are secured or not; it's such a simple thing to check and may save you some time and money. Also check inside the spare wheel well to see if there is anything in there that is loose that could be causing a knocking noise when you hit bumps or take corners or brake and accelerate.