This is generally okay with tubes as long as it is low level, but you can do some testing if you like to be sure it is a tube issue. Try swapping the tubes between channels. Start with your CDP. Take the 2 tubes and switch them. If the noise moves to the right channel then you have found the problem.
After switching the CDP tubes, if the noise stays in the left channel repeat the process with the preamp tubes. Your preamp may have a couple of pairs of tubes so you can start by switch all the tubes and seeing if the noise moves to the right channel. If it does, switch 1 pair of tubes at a time back to their original positions until the noise appears in the left channel again. At this point you have found the culprit.
After switching the CDP tubes, if the noise stays in the left channel repeat the process with the preamp tubes. Your preamp may have a couple of pairs of tubes so you can start by switch all the tubes and seeing if the noise moves to the right channel. If it does, switch 1 pair of tubes at a time back to their original positions until the noise appears in the left channel again. At this point you have found the culprit.