You mean this ventilator? There should be two of them...left and right. 
The commercial fridges do that so there is less warm air to cool back down. Think vacuum...less air mass. This makes it more efficient on keeping the cool items cool.
I think in part that the sounds yer hearing are from the air hitting the gaps in the door. Air rushing over the door will create a low pressure area and it will try to pull the door out. If you seal that gap with some tape and go down the road again...you will see how much that noise dissapears. The other item that can create that sound is the antenna and the mirrors.
The commercial fridges do that so there is less warm air to cool back down. Think vacuum...less air mass. This makes it more efficient on keeping the cool items cool.
I think in part that the sounds yer hearing are from the air hitting the gaps in the door. Air rushing over the door will create a low pressure area and it will try to pull the door out. If you seal that gap with some tape and go down the road again...you will see how much that noise dissapears. The other item that can create that sound is the antenna and the mirrors.