Speaker rattling / vibrating

[Brigand]

I'll stick this here as although its a car speaker I'm referring to, a speaker is a speaker and there may be a guru in here to help!

One of the aftermarket speakers on the rear shelf of my car has started to rattle/vibrate when there's strong bass going through it. Its been fine for quite a while but this issue has sprung up recently. Its not rattling in its enclosure either, as it's been in two different cars in two different enclosures / frames, so I've ruled that out.

I've noticed if I push on the terminals that are mounted on the speaker the rattling will disappear for a while, but will soon return. I've checked for any loose parts on the unit itself and everything is tight, there's no damage to the cone that I can see, so does anyone have any ideas what causes this?

Thanks.

[NDA]

Are you able to touch the cones directly?

If so, very gently move them in and out with your fingers and see if you can feel any scratchiness.