Upgraded to 4.0.4 audible click/pop when loading track on other deck (RZ)

This was apparent on 4.0.1 but nowhere near as bad and I stupidly thought it was ok to upgrade to 4.0.4

All faders down except 1 with track playing. Select a file for any other deck and load .. click/pop/glitch through the master and recorded into WAV

CPU meter nowhere near 100% - tried different latency from the standard one to increasing it to 7ms - no help.

Uninstalled EVERYTHING, RZ driver, RekordBox, restart, reinstall from fresh downloads - still the same.

This is on a brand new Windows 10 laptop Core i5 2.4GHz 8GB RAM 256GB SSD ... not exactly a slouch.

Any ideas on this one, its basically made my new RZ a doorstop :(

Thanks