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

If it’s a mechanical noise, it could many things but one of them is unsecured wires vibrating from the air movement due to the port.

If it’s a "flutter" then it could be either chuffing or an air leak. I once had a small air leak underneath the flange of a port and it sounded almost like the wingbeats of a huge insect.

Duke

I’ll give it a closer listen this evening. My first impression of it was that it’s the drivers working hard. 
They are 3 way bookshelf speakers (Phase Technology PC 3.5).

You have a stethoscope? It will tell the who mechanical story.

LOW listening ONLY!!!, move the scope... you'll be surprised what you can tell with one..

BE AWARE. it is not for any type of loud listening. Leaks, VC rubbing, warped drivers, loose parts, anywhere. Use a tone generator to isolate,
certain tone specific noises, with a SScope..

Regards

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.