Right channel distortion

I've noticed a problem recently with an intermittent static/distortion sound in my right channel.The cds are clean and play without any problems on a portable player. Plus it doesn't occur with every cd. I'm used to hearing clicking and mistracking if the cd is dirty but this sound i haven't heard except when my stylus hits a particularly grungy area on vinyl.
Is there any chance this could be my tube preamp calling out for replacement tubes? I've heard that tubes get "noisy" at the end of their lifespan. Is this the type of noise and could it be somewhat intermittent or only audible with some frequencies? If it is the CD player, then why isn't it occuring all the time?
I unfortunately do not have an extra preamp to test out and don't have a way to plug the portable player into my main system.
Any suggestions on what could be most likely would be greatly appreciated.
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.
Paul's suggestiions are very good. Also be sure that your cables & jacks connections are clean - do you use contact cleaner? You can either take the shotgun approach (all at once) or if you really want to know what the cuprit is/was, then do these things one at a time, & then run the rig for a few days before trying something else if you haven't yet cured it. Good luck!
The problem you describe sounds like a weak connection as in a cold solder joint or dirty tubes pins and sockets. The cold solder joint would be in a DIY kit so I doubt that is the problem.

As already stated move the tubes and see if the noise follows the tube.

Second get some very fine wet dry sand paper and clean the pins on the tubes. If you can, clean the pin holes in the sockets. I find a very small round hobby file works great, of course with a plastic handle. Put it in the hole gently and turn it. Not very much.

Another area is a volume control although this would show up when it is turned. Selector switch. Some say use a non residue contact cleaner. My experience is this makes them noisier. I find WD40 works great although it leaves a residue it is non gummy and lubricating.

Just some suggestion.
You really have not given enough information for anyone to effectively help you yet so would you try answering the following and repost so the members could make some intelligent recommendations. First, does the noise come from both speakers? If your preamp has a headphones out (some do) do you hear the noise there? Does it happen when you are changing volume? Have you reseated all of your cable connections? Have you noticed any tubes glowing slightly different? Have you reseated all of your tubes? Lastly, does it happen only when things are warmed up?