buzzing speaker - loose mount

I recently installed some new speakers in the front of my octavia and the passenger side one has started buzzing, it appears to be the mount the speaker is attached to. the speaker is screwed in tight and if I push on the mount the buzzing stops. I'm just wondering what the mounting is attached to as it does not appear to be attached to the door card. I plan on invesitgating at the weekend but I was wondering if anyone knows how it fits together before I start dismantling things.

The speakers are slightly larger than the original ones and I suppose they could but just touching something and vibrating but the drivers side is fine.

Any ideas?