NAD! Bogner Atma 18! Please ask any questions you have! : r/GuitarAmps
My amp (Bogner Atma) makes these noises whenever it's switched on/off. The little research I did leass me to believe it's a tube issue (specifically power tubes). Is this the most likely
Bogner Mini Ecstasy: A mini head with 30 Watts of solid-state power - gearnews.com