I was actually able to get a similar result like FZ at 48kHz with my UFX. I also testet it with my trusty Behringer ADA8200 slaved to the UFX via ADAT
With nothing connected to both preamps I get peaks/interference at 3,4kHz , 16,8kHz, 20,3kHz and 23,7kHz on Inputs 11 and 12 of the UFX with 55db of Gain. I get a bit of 20,3 on Input 10 and visually no interference on input 9. The peaks on input 11 and 12 are around 5-7db above the noisefloor in each frequencies range. I do not get any peaks on the Behringer, even when nothing is connected with gain maxed out. The noisefloor is about the same on both units with these gain settings. (-66dbfs peak). I measured both preamps with a test signal into a Beyerdynamic M201 to put out equal level at these settings.
With the M201 connected to the UFX all of these peaks disapear and the noisefloor drops 8db at 10kHz to about -102db (low end rumble in my room make a fullrange peak measurement irrelevant). With the M201 connected to the behringer nothing changes, but the noisefloor drops about 9db at around 10kHz at -103db.
Then I connected a Rode NT5 SDC to both my UFX Input 12 with 55db of Gain and then to one of the Behringer ADA 8200 Mic Inputs with the Gain maxed out.
With both Preamps I get interference at 18,4kHz.
I noticed the level of Interference changes when I change the direction the Rode Mic is facing. This happens with the M201 aswell. After these tests I kind of get the feeling that both FZ and I might be dealing with some kind of HF interference from outside sources, here. But I am no expert in this field.
If this was the case however, I am wondering why the Behringer did not catch anything when nothing was connected.
Both units are directly mounted in a rack together (Don't hate me ) and share the same poweroutlet with a Brennenstuhl Premium Line Powerstrip.
I would appreciate any knowledge on how to get rid of this.
Ps: I was also kinda shocked to have slightly lower noise in the high frequencies on the Behringer ADA8200. In the case of the Rode NT5, the mic is obviously limiting the SNR. But with the M201, I expected the UFX to be quieter than the Behringer.