Crackle from tube preamp

Hello. Need troubleshooting advice. I have noticed a faint intermittant crackling/popping noise coming from mostly one channel through the speakers at 18 inches away or closer but sometimes the other speaker too. The unit seems to exibit a fair bit of hiss anyways. It is evident a low volume and pronounced slightly with increasing the volume. I have switched several types of tubes now and thought originally that a tube was bad. It is definately coming from the preamp as when the source (also tube) is unplugged, the noise is evident at varying intervals. It diminishes a bit when the unit is warmed up over a period of time. The music is not affected or "broken up" on playback. I would expect to hear a slight constant hiss from both channels with proper function ni a tube preamp. Might it be the fuse at the back? Maybe a capacitor? Tube sockets? Not into the guts of units, just like clean, nice sounding music. If it sounds familiar, can you make some suggestions? The unit goes through an Audioprism power conditioner at the moment, along with several other components. We live rurally. THanks. Dave