The others have hinted at this, but tubes can be microphonic. The glass shell can vibrate with the sound in the room and this can get amplified again. Some brands or types of tubes are worse than others in this regard. Normally one would expect any problem to increase with playback volume.
An easy test is to tap very lightly on a tube with a pencil and see if you hear anything through the speakers. If so, tube dampers can help with this issue.
An easy test is to tap very lightly on a tube with a pencil and see if you hear anything through the speakers. If so, tube dampers can help with this issue.