Could be the tubes. Turn the preamp completely off and switch the tubes from one channel to the other. If the noise moves to the other channel, then its likely the tubes.
Could be your cables. Try taking the interconnects off and putting them back on making sure they're tight. Or try a different set of interconnects. And make sure your speaker cables are securely fastened on both ends. (Actually I would do these things before messing with the tubes.)
If none of that works, it's likely your cd player. You probably can get an adapter plug to plug your portable in to the main system, unless its a boombox type.
Could be your cables. Try taking the interconnects off and putting them back on making sure they're tight. Or try a different set of interconnects. And make sure your speaker cables are securely fastened on both ends. (Actually I would do these things before messing with the tubes.)
If none of that works, it's likely your cd player. You probably can get an adapter plug to plug your portable in to the main system, unless its a boombox type.