Thanks, I recall that in the manual but I always thought that MK2 was compatible with everything and being the thicker type was less susceptible to noise.
You would think so, but alas, not: Either the "spare" wires in the Mk.2 cables are picking up the noise, or else the Mk.3 cable is adding a ground. (Not specific to BL3; e.g. BV10->BL18 requires Mk.3 (4-wire) because Mk.2 (8-wire) makes that "data" chirp on volume up/dn.)
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 +/-.
Using an 8-wire RJ45-connector Cat.8 (has both pair shield & connector ground) patch cord drove BL18 without noise,
***provided a 4-wire Mk.3 PowerLink cable was used from the BV first***, before the PL-RJ converter dongle. Both flat cable & round worked; shouldn't matter if the pair twist is to spec. Also Cat.7 should be OK if it's SFTP (shield&foil-twisted-pair) with the grounded metallic connector head. I chose Cat.8 for the safety" guarantee (higher data rates = better shielding), but that shouldn't have mattered. BTW, a (so-called) "Cat.8" female-to-female connector worked OK in that chain, too.