You can always swap speakers around and see if the problem moves with the speaker.
If it moves, then you have a couple of options.
First, you can try vacuuming the panels. Unplug power over night, then clean away. If this does not make a difference, then you can wash the panels in the shower. Just use warm water from the shower head to rinse them off - no soap or agent that may affect the coating. Then let dry in front of some fans blowing over the entire panel.