What do the different sound triggers mean

[Bear]

for example the howl is clearly Bigfoot but what is this popping almost grenade launcher like sound

[LT1956]

Big howls to detect you, he can SMELL you scent for about 10-15 seconds, that how he knows were you always are.

The Knocking sound that some describe as Shots or Grenade sounds are Bigfoot Knocking on the trees. Sometimes to show he is near or ready to attack. The knocking is done in code and the BF doesn't actually hit the trees in the game, though maybe they will change this later. Turning on Noise like Radio and Siren used to cause bigfoot to investigate in 3.0 this is not confirmed to be active in 4.0