Not sure of term but I notice some distortion/vibration coming out the speaker port?

First off...I know that you don’t listen to speakers this way! 😜

However when listening to my speakers at a decent level if I go the the back of them in one speaker coming out of the port I hear a little vibrations/distortion sound?

I cannot hear this when listening at my MLP but wanted to see what you guys though. 
Does it sound like my speakers bolts need tightening?
So I need more wadding?

Am I listening to loud?

You said one speaker. If its the same from both its normal. If its only one first reverse channels to be sure its the same signal and level you're comparing. Then if its still only one look to see what it is. Loose speaker is very unlikely- you would hear it from in front not behind. Duh. It could be anything from loose or cracked port, to filler that came loose and is flapping against the port inside. 

Whatever it is, if the only way you hear it is to stick your ear back there then my advice is the same as Henny Youngman's doctor gave him when he said Hey doc it hurts when I do that: "Don't do that."
I took my handy flash light and looked into the ports. 
The speaker I’m hearing this in it appears the wadding has shifted and it almost blocking the port. 
The other speaker wadding is still intact. 
I’ll pull the speaker terminal block off tomorrow and move it back. 
It's the Port Noise complaint. (Is that a somewhat dated pun? I'm old...sorry)