cbauer,
If you have some Deoxit cleaner solution, it works well as it cleans and conditions a little (not Deoxit Gold...this is mainly for conditioning after cleaning and I think tricky to use without getting too much on). Input tube sockets are so little they are hard to get anything into. I got some of those little tooth floss/brushes from the CVS (red and white handles with a sort on miniature soft bottle brush). They are small enough to get between teeth and work pretty well in small sockets. 99% alcohol works in the place of Deoxit, and some think it is better...no conditioning, and can probably be gotten locally.
In a fix, I just cleaned the tube pins first, then sprayed a little deoxit on the pins and gently worked the tube in and out of the socket with some side to side motion along with in and out. It worked.
Could be a bad tube, or just dirty pins. So cleaning is a good thing. After cleaning, if you still hear it, you could check the bad tube thing by switching channels and see if the racket moves to the other speaker. Also maybe clean pins and socket once more.
I finally bought this
http://www.musicdirect.com/p-1158-caig-vacuum-tube-survival-kit.aspx but have not used it yet.