Weird buzzing issue...

I was playing at low volume and was hearing a buzzing that I thought was coming from the cab I was plugged into, so I switched cabs. Buzzing was still there. Changed amps, buzzing still happening. So I'm having a *** thing going on. Plug back into the first amp and cab, and kneel down in front with my head right up against the grill cloth and play. The buzzing was definitely NOT coming from that cab/amp. So, I'm really scratching my head, because when I stand up and play, I hear the damned buzzing sound... I finally tracked it down to a Marshall 2x15 cab that was never plugged in to begin with. I had just swapped speakers in this cab, replacing the JBL 2225Js that were in it with the original Celestion Sidewinders it came with, and when the cab had the JBLs, there was no buzzing coming from it. (still following me?), so the cab itself isn't buzzing. It's one or both of the Sidewinders. I plugged into that cab, and it became very obvious that the buzzing was coming from it. The speakers both work, and measure correct DC resistance (16ohm impedance, measuring around 13 ohms DC, wired in parallel for total cab impedance of 8 ohms).

So... the reason for this post. What on a speaker could be loose so that the speaker still works, but makes a buzzing noise, even when it's just being vibrated from outside the cab itself?

Thanks for any insights.

Cheers!