It should be easy to tell the difference between sheet metal and a HV arc- they usually sound rather different.
I am curious what you find because I am having a similar problem with my AL1500:
After about 10 years service, one of the HV caps failed: there was a bang and the fuse blew. When I checked the capacitors, it was obvious one had vented. I replaced all the caps and bleeder resistors.
After the cap replacement it ran fine for a couple of months. Then completely at random there was another arc somewhere in the HV supply. I did see a flash through the vent holes on the left side of the amp, but with the cover on I couldn't locate the exact spot. Very loud, definitely not a sheet metal noise. This time the fuse didn't blow, but the protection circuit took the HV off line.
I took it apart again, but couldn't find any obviously damaged capacitor or bleeder resistor. I also discovered the arc had destroyed one transistor in the QSK circuit. I replaced that and put everything back together. As soon as the HV turn on, another big bang
So now I need to take it apart again.
Maybe I have a wire out of place that created the arc. One clue is that when it arcs the fuse does not blow like it did when one of the caps failed. But testing is difficult, because every arc takes out more components in the QSK board!