Why are my speakers resonating????

Hi and thanks for your time. This is my first post although I have been with Audiogon for many years. I have a pair of MB Quart S802 speakers a few months ago and with certain lower frequencies of music I.e., piano, acoustic guitar and female vocals I have one speaker that resonates at times during these lower notes? These are replacing a pair of 1995 MB Quart 502s I purchased new and they never gave me anything but pleasure. Nothing back then came close for their price and I sampled a lot of speakers. I never understood how anyone could say their "too bright" either. One person wrote it in his review and it still sticks today. I run all Acurus amps and their AC3 processor, an NAD 565BEE c.d. Player along with all Audioquest connectors, NRG5 power cords, CV8 w/72v spkr cbl,, King Cobra audio cbls and VDM5 digital cables.
Any help is very much appreciated. I do like the Quarts. I contacted them, now Maxxaonics?, and their not able to tell me anything because they purchased Quart after the 802s were made in 2006. And yes I wish I could get some Von Shweirts' or 802 D's even PSB but the last 15% of a restoration on a 1966 Ferrari 275 GTB4 my father and I were building and now I am the soul financier of it as he passed a few months ago.
Check to see if the screws holding the speakers in are tight. If that's not it try swapping speakers from side to side to see if the problem switches sides.
The first thing I would do is exactly what Sarcher30 recommends. Drivers come loose all the time. When it comes to swapping the speakers, though, I would do it both ways. First, just swap the speaker cables. If the problem now moves to the other speaker, chances are the problem is somewhere else. If the problem is still there, then move the speakers. The reason for doing it this way is that you may have an issue with the room that may fool you into thinking its the speakers. Switching the channel and then the location gives you 2 separate verifications as to whether you have a speaker problem, or not.

After doing all of this and you still have a problem, can you better describe the sound you are hearing? More details will help you narrow the problem down to a solution.
Agree #3 with check screws holding speaker cone.
Tighten snug, but do not strip out.