I had a similar issue with an amp a few years back. It turned out to be a solder joint at the tube socket that had broken. When cold there was enough of a gap to cause a problem. But when the tube (and socket) warmed up, heat expansion closed that joint and things worked fine. While it's making noise you may try wobbling the tube lightly in it's socket to see if that alters the noise at all. If you have any spare tubes, put them in to see if it's actually the tube. If not, then start looking over solder joints. If they all seem fine, then check the sockets themselves and then I'd start checking tolerances of components while cold.
Best of luck!
