Things to try;
1. In case you recorded a one-sided stereo track use: Edit, Track > Split stereo track into 2 Mono tracks, delete the blank track.
2. Press the Stereo/Mono button to the left of the MIX knob on the UMC204HD.
3. In RealBand, on the track in question, slide the L/R vertical slider to the center (or Right Click on the slider and use the dialog box to enter O, Click "OKAY".
4. See if you Main "ALL" knob is panned to one side in the Mixer Window. (hold Ctrl and press 8 to open the Mixer window)
5. Check your audio devices properties to see if one fader is down or if the balance is to one side. Like this; In RealBand, bottom right corner, Audio Settings, Playback Devices,click on UMC204HD so it is Hi-lited, Properties, Levels, Balance, make sure everything there is at 100%.
6. It could be the way you are recording a MONO track. To check this; Options, Preferences, 2-Audio, find "Input Channel For MONO Tracks" change it to one of the other choices in the drop-down menu; record again. This is something that needs to be set before you start recording a track. Also should be sure the track is set to mono (which is most common) before you start recording. This is noted by the default single or double wave icon on the track type.
7. If you want to try to convert a one-sided recording into a centered mono track I think you'll have to export it to WAV, open it in Audacity, export it from Audacity again as a WAV. Set the next track in RealBand to be a MONO track and Use FILE, Import, I should now be a MONO centered WAV file on a track in RealBand. I offer this method because RealBand does not have the option to Convert a Mono track into a Stereo L/R Track. Which would audibly sound the same as a single mono track.

Does the noise in your head bother me ?