Loud Popping / Electrical Noise?

I have been getting a popping/electrical noise from my speakers .
My equipment is all Mac ( tube preamp ) and digital source.
Does anyone have any idea what is happening ?
Pre amp tubes have probably gotten noisy and need to be replaced. Turn off the pre amp (or mute it) leaving tha amp on. Does the noise go away? If so Its the Pre-amp.
Thank you Newbee.
I gave the preamp increrased ventilation and he popping has stopped. Go figure !!