I actually removed also the usb so no actual signal is being sent to the H9 and the hum noise is this there.
The other thing i tried is use the pedal not as send/return but as a master insert. Here the noise is gone! Even if then add the usb back so there is signal send to the H9, there is no hum noise.
The connectors for the master insert however have different specs.
MASTER INSERT SND / RTN
Provides INSERT points for the MST1 outputs to connect hardware dynamics processors such as a limiter, compressor, or graphic equaliser. Or when connecting effects hardware in-line with the (MST1) LR MIX output for external MASTER INSERT FX. Insert devices do not affect MIC A/B inputs. INSERT SND: Balanced (TRS) 1⁄4“Jack, wired; Tip=Hot [+], Ring=Cold [-], Sleeve=Ground. INSERT RTN: Unbalanced (MONO) 1⁄4”Jack, wired; Tip=Hot [+], Sleeve=Ground.
while the Send/Return 1&2 i used previously
SND 1 / SND 2, LINE Outputs
Connect your external FX inputs to SND 1 or SND 2 outputs and use SEND 1 / SEND 2 controls to send Channel audio to the FX unit. Connect MONO devices via L/M. Switchable SND 1 input and RTN A output impedance for external FX between LN, or Hi-Z suitable for guitar FX pedals or legacy hardware.
Balanced (TRS) 1⁄4“Jack, wired; Tip=Hot [+], Ring=Cold [-], Sleeve=Ground.
RTN A / RTN B, LINE Inputs
Connect auxiliary LINE level music sources, or external effects device outputs to RTN A and RTN B LINE inputs to route to CH A and CH B. Use L/M sockets when connecting MONO and Hi-Z devices. Balanced (TRS) 1⁄4“Jack, wired; Tip=Hot [+], Ring=Cold [-], Sleeve=Ground.