Hi all, first post so please be gentle 
I have Phonak hearing aids and use Bluetooth to connect to my mobile. All works well until I open apps such as Facebook etc. The aids are producing a hissing noise even when no sound is playing but as soon as I close the app, after a couple of seconds the hissing stops. Any advice would be greatly appreciated.
Thanks 
Phone is a Blackview BV9000
They’re connecting to Bluetooth, so switching to one of the Bluetooth+Mic programmes. You can’t turn off the Autoconnecting to Bluetooth, but you can reduce what connects.
You can go into Bluetooth, and switch off Audio, and just leave Calls as the only connection with your phone. I’ve generally used this approach, and when I want to stream, I go into Bluetooth and switch Audio on.
The other approach, I’ve recently learnt, is just to switch off automatic video playing in Facebook. This also eliminates most of the autoswitching that’s going on. Facebook will switch to Bluetooth streaming even if the volume is on zero.
Edit: As standard, your aids will startup with Autosense, which already switches between defined programmes, to (hopefully) suit your current environment, these changes can be similar to what you’re experiencing.
Peter
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And In App Sound on Facebook, then you don’t get any issues. 
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Recalling… but need to check: There’s a setting in Phonak Target (the application that your audi uses to set you up) that squelches noise via streaming when it’s nominally silent. I enabled it on my L90s and it helped.
It would be interesting to know if you are hearing the hiss in both ears. If it’s just the side that connects via Bluetooth to your phone, have the noise reduction increased slightly on that side only and see if it goes away.