Difference between BF4 and BF1 armor
Unlike in BF4, where only AT weapons dealt any damage to tanks, in BF1 most weapons deal some (albeit little) damage. Regular frag grenades, incendiary grenades etc. In fact the recent Fall Patch has increased the damage regular grenades deal to tanks.
Another thing is that unlike in BF4 where rear-armor was a tank's weak spot, in BF1 it's the tracks. Tanks can be easily immobilized if attacked from side.
AT weapons
All classes can equip light AT-grenades, which deal much less damage than heavy AT-grenades, but are lighter, and thus easier to throw farther.
Once you level up your assault, you'll gain access to AT mines, as well as the AT "rocket gun".
Snipers have K-bullets, which are specifically designed to damage armor.
Anti-tank rifles spawn as a battle-pickup (for example there is one at E flag on Sinai Desert Conquest).
Field guns and fortress guns also do high damage to armor and can be operated by any class. Their downside is that they're stationary.
HE mortar can do significant damage to tanks without the need of exposing oneself to direct fire.
AT mines, which can be dropped either by Assault or Artillery Trucks are extremely effective - 3 mines can currently take out any armor in the game.
And of course, tanks are vulnerable to bombs dropped from airplanes. The Attack Plane can be equipped specifically for tank hunting.
It looks like you can only view it online, not in-game. See here:
You can access the current test version at the following address. (EA login required)
https://www.battlefield.com/companion/career
You can view your stats from BF1 and BF4, see your kills per weapon/vehicle, edit your infantry/vehicle loadouts and check your progress on medal assignments.
Best Answer
I found a video that discusses these sounds and the possibilities behind it:
One theory is that the sounds are suppose to be reenacting insane soldiers. In World War I, some soldiers in the field demonstrated what was called Shell Shock, which is a type of reaction to being helpless during bombardments and war in general. It is a form of PTSD (though PTSD wasn't a term yet at that time). Some men illustrated mental breakdowns from this disorder and had to be hospitalized. To relate this back to the question, the door that the sounds are heard as the world "Isolation" above it, which helps further back this claim.
Another theory as you've said is that it is zombies or it is a continuation of another Easter Egg in the game. The Easter Egg in question is the Marconi Easter Egg, which appears to be a lengthy Easter Egg in Battlefield 1 that involves you going across all the multiplayer maps and listening to Morse code. You unlock a dog tag for doing the Easter Egg, and after doing so, the telegraph stations will then be on a weird audio loop that some find rather creepy. This could be a continuation or just a part of this Easter Egg. It would make sense though to keep zombies in an "Isolation Room."
The last theory is (as you've also said), walrus noises. The linked video above shows a comparison between the sounds from the door, and the same Walrus video that the noises are speculated to have been pulled from. Admittedly, they do sound extremely similar.