First and foremost, any digital servo worthy of the title should always be making some noise holding center if there is any weight on it. If, without anything connected to them, your flight surfaces were perfectly balanced where they would remain in absolute neutral, then, and only then, would a digital remain quiet. The signal sample rate for a digital is much, much higher than an analog, and it should do everything in it's power to perform the task it has been assigned to do.

Second, installing a PCM rx in the attempt to mask or reject an internal issue that you are already aware of is begging for disastrous results. PCM should be used for the rejection of minor stray and outside rf issues, not pre-existing problems. When a PCM rx goes into failsafe mode, it doesn't care where the interference comes from. If the interference comes from inside the plane, it can, and has, failsafe all the way to the ground.

Bottom line, if you know you still have a problem, find and fix that problem before you fly the plane with any receiver type! If you are getting a good range check, based on the radio manufactures suggested range test proceedures, then you should be ok. The difference between the range test differences with a PPM receiver should not be more than 10%, engine off and engine running. Leave the PCM rx out of the system until you can meet those requirements.