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.