That trackpad flex is normal considering the huge size and that is a thin piece of glass.
Tried it myself on 15" 2017 and 2018 MBP and it's exactly the same, so that's the way it should react to hard press in centre of it.
If i click it normally or even hard click it for the second click, still doesn't flex, so it requires some extra force to flex like that.
About the sound yours makes, that's not right at all.
Edit:
Found a better explanation for the trackpad flex:
"The new force touch trackpad is suspended on four springs on the corners, rather than being hinged at the top like the older design. Strain gauges determine the forcefulness of your click by measuring the deformation of the trackpad.
So YES, it does flex down. It is NOT the same as an iPhone's force touch (which does not flex).
No, that is not a defect -- it is essential for measuring the amount of force applied and allowing the click-anywhere feature."