Distorted kick decay noise on 1031A

Hello friends,

I have a pair of Genelec 1031A in a studio in Romania, bought second hand from a previous owner. They worked perfect for about a year and then a problem started appearing on the left speaker. It can be heard after the transient attack during decay, as a lingering buzzy noise. It's not appearing when I play music without a clear deep kick. If I create a track with just a kick, it's very obvious even at medium-low volumes.

In general it mostly appears at medium to high volume, I tested properly to eliminate interface problems, cabling, etc. 

How can I identify the problem to resolve it? I have by the way a set of new driver boards acquired on Reverb but am wary of opening the speaker for replacement, without certainty even.

Please advise. Thanks,

Eduard