Yes, could be either of those or you could have tracking. If you can arrange a shutdown do it as soon as possible. A visual inspection of all insulating surfaces should be done looking for signs of tracking, use a black light for the inspection that will make it easier to find any traces of nitric acid. If you cant shut down a visual inspection with a corona camera would be best.
Surely, listen for popping, or cracking sounds produced by electrical discharges also humming sounds or vibration produced by quality. With lights out, look for blue or purple corona halos. Orange or red sputter arcs are created by intermittent sparking. Best way is you cannot blackout a graphic checkup with a halo.