Possibilities - 1) Tubes are not 'low noise tubes' and your amp requires low noise tubes; 2) Speakers are too efficient and reveal the noise floor of your amp; 3) A combo of 1 & 2, 4) Hearing a volume control when it rotates (I assume you are talking about the mechanical noise from the amp, not the speakers) is not really rare and doesn't hurt anything, but if the sound is bleeding thru to the speakers and is quite noticible I'd have the amp checked out. Maybe it just needs cleaning.
I'm sure there are other possibilities but these come immediately to mind, the first being the most likely.
I'm sure there are other possibilities but these come immediately to mind, the first being the most likely.