Hissing persists after restarting the unit with no other app running except Ibus app. Shutdown Ibus app and hissing stops. Contacted Avin and response is "How to fix ground noise. Use a wire to ground the metal frame of the radio to a bare metal part of the car."

I'm yet to try the above but did anyone do this during their install?

All indications on my end is that the Ibus app is the source of the hiss. It's a constant hiss (sssssssssssssssss) that does not change with vol, happens whether engine is on or off, vol level has no effect but stops when at 0 level. My workaround now is to turn ignition on, wait for the HU to finish booting and Ibus app starting up, turn off ignition and immediately turn it back on. Hissing sound then disappears.

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'05 E53 X5 4.4i, '97 E39 528, '07 E92 335i, '16 F86 X6M.