Preamp noise

Is a slight amount of noise coming directly from a tube preamp 'normal'?

I've owned several tubed pieces -- Fisher 800-C (receiver), Audio Research LS-7 (pre), Juicy Music Blueberry Extreme (pre), Modwright SWL 9.0SE (pre). And I'd say that yes, a slight amount of low-level tube hiss is normal. Some tubes are quieter than others, though.

The sound is more like a hum than a hissing sound.

It sounds like you are talking about mechanical hum conducted through the air. If so, is your power transformer mechanically decoupled from the chassis? This is usually done with rubber grommets. You could try loosening the transformers mounting hardware to see if this changes the hum at all.

I missed the "coming directly from" in your initial post. That does suggest that you're hearing this directly from the pre-amp rather than through your speakers.

I've also found that a light hum from the power transformer isn't unusual -- even with solid state equipment. Sometimes this is caused by loose transformer mounting hardware, as stated by jbeede. Or it might be caused by dirty AC power -- e.g. light dimmers adding DC voltage on your AC power line. It could even originate from one of your neighbor's homes. And there's probably many other causes that I'm not aware of!

Well...I know the wiring in this condo sucks. I have the amp and preamp both plugged into an industrial line conditioner. The hum directly from the preamp isn't getting louder. I currently have the cover removed and I bet when I put the cover installed the hum volume will be less noticeable if not completely inaudible.

The dc bias supplies in my electrostatic speakers hummed--the sound actually kept me awake at night on occasion. I removed the mounting hardware, slightly enlarged the mounting holes, inserted rubber grommets and reinstalled the transformers with slightly smaller hardware. Dead silence. End of story. Do it.