Others have provided helpful tips on isolating actual panel-sourced issues.
But if those prove the panels are good, I have another angle for you to explore, as I encountered this as well setting up my Monoliths.
I grew up in a household with a Steinway grand piano played by a Julliard-grad pianist, so I have a very keen reference for piano acoustical performance in many venues. It's the main reason I bought into MartinLogan's 27 years ago.
Certain individual piano notes or chords readily excite specific resonances on panels if they are not well anchored or have some defect as noted above, but they also energize room modes as well, which are often confused for equipment issues.
The larger the panel, the more energy they put out (again, a dipole emits equal energy to the rear as the front, so that needs to be factored as well) and can cause 'ringing' within the room based on construction and treatment. Give your Norweigan location, odds are you have solid concrete walls, which are very reflective and unyielding at any frequency, thus contributing to maintaining resonances if excited.
Even in a room custom-designed for ESL deployment (I built my home after I bought all my ESLs) with all the magical ratios and space to locate them out from the walls, I discovered huge issues with resonances with bare walls. Solo piano was painful at moderate to loud levels. From about 2Khz on up, resonances would zing like crazy.
I embarked on a decade-long process of discovery with measurement tools and ever-increasing acoustical treatments, most documented on this site. Every addition resulted in reductions of certain resonances, to the current state where they are mostly managed and I can play solo piano at close to realistic levels and enjoy it for hours.
A big part of the challenge is actually managing the rear-wave reflecting and causing comb-filtering, which on the frequencies where it's additive further contributes to resonances. Bottom line, place 4" absorption behind the speakers and on the side walls between the speaker and front of room to mitigate side-wall reflection of the rear-wave.
I often see ESL deployed into pristine modern concrete wall rooms, with tons of glass, refelctive floors, and zero acoustic treatments and my ears start to hurt just looking at the pictures, as I know the acoustic hell that must be.
