Help - Tube amp Problem

Hello there
I need help.
Problem: One of my KR8000 mono amps (Vacuum Tube Push-Pull) is making my speaker do a weird intermittant noise (a bit like a small firecracker: prack..., prack....,
etc..). Though the music plays, the noise makes it difficult to concentrate on the music.

Even when only the speaker is connected to the amp and no other gears connected, when the amp is powered on after some time the noise (prack prack ...) starts. When this happens, I can see the bass driver of the speaker moving in and out quite violently as if it would burst.
I have tried different combinations of cables, UPS, with and without CD,etc. and am sure that the amp is at fault.
The amps have only about 500hrs on them.

Because of the weight of the amp, sending it over to the factory would be very expensive. I live in Mauritius.
I would appreciate your help in identifying the source of the problem and guidance on how to fix it.

It is very likely a defective tube.

Unless you just want to buy a whole new set, the conventional approach is to swap tubes from one amp to the other until you find it. First swap half the tubes, maybe output tubes. If the problem moves it is in one of those tubes. If it does move, swap half of those back to the other amp and so on until you have identified the culprit. Divide and conquer!

You could also try turning off the lights in the room and just observing the tubes when the noise occurs. It is possible you will be able to see the bad tube arcing.

Good Luck. BTW, my neighbors are from Mauritius.
thanks a lot. I swapped the tubes and the problem shifted to the other mono. This definitely is a tube problem.

I then installed a brand new pair of tubes and listened for 2 hrs without that noise recurring. But I could hear some very small noise (at the very first time the new tubes were played) on the mono which accommodated the new tubes. Maybe new tubes do that. I'll get back after extended listening during the weekend.
By the way, glad to know that Mauritius is known in the US. I worked at the World Bank in DC and hardly any American knew about its existence.