If your speaker makes a scratching or buzzing noise that you can hear when you gently and evenly move the cone up and down, one problem might be dirt in the voice coil gap. There are other things that can also cause a speaker to buzz, but if you have isolated it to dirt in the gap, then you can try to clean it out.

Remove the dustcap or center dome, and insert a piece of tape in between the inside of the voice coil former and the magnet pole piece. You must inch it down the former a small amount at a time and then rotate 360 degrees to try to get the loose dirt to attach to the tape. Gently remove and try again.

This will work for debris on the inside of the vc former. If there is debris on the outside of the former, you can try to evenly lift the cone to the highest excursion point and then use the tape in the same way.

If you cannot remove the debris, then the speaker will need to be reconed so the gap can be more thoroughly cleaned.