A couple of things to troubleshoot, to make sure we get those out of the way, first. First of all, is there a microphone, or anything else that requires phantom power connected to the preamp that the 48v phantom power is applied to? If so, does that microphone need to be on and plugged in, or are you planning on using it, immediately? If there is no need for it, immediately, deactivate the phantom power and disconnect the microphone. The noise should go away. However, it you DO need the microphone (or device that requires the phantom power), you might want to consider putting a noise gate on the track that the preamp is assigned to. With a noise gate, you should be able to get a handle on the background noise generated by the preamp. In my set up, I find between -18dB and -20dB is sufficient to squash the static of phantom power (and the other ambient noise that can be found around my home studio). Good luck!
