Hello , i have a line6 hd500 plugged into an IRT studio via the 4 cables method...the fx loop produces noise just like ground noise...it is not the pickup noise since a noise gate does not help..even when I unplug the guitar I still hear it....I dont have such a noise when plugin the hd directly in the amp return ..adjusting the fx block signal in the fx block of the pod does not influence the behavior of the noise ... the pod is set up to amp not to line and is in stomp boxe mode..... any suggestion ??