Testing an Ampeg SVT-3 Pro and have the following behavior observed: * Nothing connected other than AC and speaker * Master volume off * Power on unit * Wait for power up delay * Hear relay in amp switch on * Hear an brief but audible POP! through the speakers with a quick decaying hum similar to an old tube TV when you first turn it on * POP! noise does not increase with master volume control * Otherwise the unit seems to sound and function properly - no noise, crackles, power offs, odd sounds, etc. Some further troubleshooting: * Remove tube from V1 * Repeat process * Still hear the POP! * Engage MUTE button * Repeat process * Still hear the POP! * Plug patch cable into power amp in jack * Repeat process * Still her the POP! I've seen some other threads where someone suggests DC offset issues in the power section, others where they say the relay popping noise if normal.