The more components you put in the signal path the more likely you are to get a ground loop. That is because each has to see the same ground plane to prevent creating a situation where one or more components bleed their voltage to ground at a slightly different rate which results in a voltage potential between them. All A/C components bleed their voltage to ground in the main box where the breakers are located. Both the ground plug and the wide prong on a two prong component end up at the same place in the breaker box effectively creating two ground planes if not electrically the same coming out of the receptacle. Grounding the two components together will eliminate the noise issue in most cases but the are other ground planes being seen by your system too. The cable most of us have coming in from either a networrk or the cable company/Directv is also grounded somewhere which creates a third ground plane which not all of your components see. Usually the only components that see it is the cable box and the AVR/AVP. As an experiment disconnect the cable box from the Marantz and see if the problem goes away. There are cheap isolators that can fix the issue if that is the cause.