I dont think it is the cables – or rather it is (silly)
I had the same issue taking a regular CAT7 (flat cable), connecting my Beolab3s (with RJ45 to PL connector) and the issue was bad. I could clearly hear an impulse when pressing vol +/-.
I threw out all my PL cables and RJ45’s and replaced the lot with Sound Heavenly cables. It worked but there is still a noise.
Im not sure of other peoples setups, but I think this has a good deal to do with the number of PLs connected (3 pairs of BL3s) and the way Theatre controls the signals.
Its workable, almost imperceivable unless your ear is against the driver but it is there.
To summarise,
- Have good PL cables to improve cable hygiene
- Theatre may have a PL distribution issue.