Noise from Left Channel

I noticed last night that there is some noise or low level hissing coming from the left channel mainly the tweeter. I am wondering if it is a tube from either the CD player or preamp. Both of them are Jolida. I seriously doubt it is the amp because all of the caps were replaced just a few months ago and thoroughly gone thru with a fine tooth comb. Guess I am asking what happens when a tube starts to go? Thanks in advance.
Can be a faulty tube - but usually a faulty tube will let the 'infected' channel go softer till it is totally quiet.

Don't be surprised if it is a capacitor! Also suspect loose connections & dry soldier joints/connections in the amp or cdp.

Kind rgrds,
It has not got any worse in the past 2 days. Hmmmmmm.
A standard method to find out if a tube is at fault is to switch the tubes in the left channel to the right and vice versa. If the noise switches sides then you know it is probably time for some new tubes.

Two caveats. First, sometimes the noise is present because the tube is seated poorly or some pins have some corrosion. Removing the tube and reseating should cure this problem. The second caveat is only work on one device and one pair of tubes at a time. If you change too much at once you won't know which exact component is involved.

If the noise does not switch sides or disappear, then you know it is not the tubes.

Keep in mind that just because the amp was serviced recently does not preclude the caps or any other part. No matter how tight their quality control, every manufacturer will have a few parts that seem to pass but contain a defect just underneath the surface and fail sooner than they should.
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.
what happens when the amp is on and cd player off? do you still have the noise? if yes then its the amp. otherwise its a cd player. try switching the tubes from left to right channel on the unit in question and see if the hiss will shift to the right
Thanks to all for your suggestions :) I will do the tube switching soon.