If I were guessing, I'd suspect that the 11 kHz noise you hear is an artifact of the encoding process interacting with the self noise of the microphone. Just a gut reaction, though, and I could easily be wrong.
On the 50 Hz noise, let's go with a decision tree approach:
1. Does it hum with other condenser mics? If not (as I believe I read), then we can rule out a lot of issues right off the bat. Any hum in the phantom power would show up as long as phantom is on because it would go directly into the input. Any noise later in the audio chain would not change when you change mics. This leaves three possibilities:
A. The microphone itself.
B. The mic cable.
C. The preamp's case ground.
2. Does it hum less when you touch it? If so, then either:
A. The set screw that holds the XLR connector in place is loose. Tighten the screw(s).
B. The microphone cable has a poor ground. Clean the contacts. Tighten the screws on the cable.
C. The microphone cable does not ground the chassis of the mic and neither does the mic. Add a jumper wire between the ground pin and the ground tab on both ends of the mic cable.
3. If not, then either:
A. You have noise on your ground (shield) and the design of this particular mic doesn't protect against that. Fix the grounding problem with a
grounding pigtail. (Try 2C first, though.)
B. You're in a really electrically noisy environment and LDCs are just hopeless.
