A microphonic tube will ring, howl or produce general feedback problems. It will be more noticeable at louder volumes or when used in close proximity to a speaker, typically in combo amps. If the tube has good tone at lower volumes and is free from unwanted noise, you use it in a less sensitive part of the circuit, such as tone recovery or phase inverter applications. Noise is more of a problem than microphonics. A noisy tube will make random popping noises, crackle occasionally or just hum.
Tubes designed for high gain can exhibit more background noise.
Other components can cause noise problems that may be blamed on a bad tube. Plate resistors are notorious for causing hiss and crackling as they age and begin to fail. A new tube may better amplify these defects, so try substituting another new tube to be sure of the source of the noise. 12AX7 - These are not relabelled Sovtek 12AX7LPS tubes. There is a marked difference in construction and performance.