Hey folks,
I just installed my F90BT, and I've noticed a very annoying hissing noise (very noticeable when volume is between 01-03). It occurs mainly after driving directions are sounded, or when moving between screens when the beep sound is created.
It does seem to go away after a while, and it is not very noticeable at higher volume levels as it gets drowned out. However, neither of these are great "solutions". 
Any ideas?
The fan on the back of the deck?
No definitely not a fan noise. This is a speaker hiss. I thought I might have a bad ground or something, but all my connections are good, and wired bypass is working flawlessly. 
I had a hissing sound coming from my speakers , so i took my d 3 out and started putting electical tape over anything the was metal near the back of the unit , now i don`t hear any hissing at all. 
just music 
I had a hissing sound coming from my speakers , so i took my d 3 out and started putting electical tape over anything the was metal near the back of the unit , now i don`t hear any hissing at all.just music
That was my guess too (bad ground). Just wondered if everyone got the same results I had. Oh well, I'm taking my dash all apart anyhow to hook up my ipod cable that my F90 didn't come with even though it said it should have... (at least I got it free). I will check the grounds on everything again, and tape like a fool! 
I just posted a reply to your other post about BT being LOUD. A line level converter will fix you hiss problem too! It did for me 
Vorian
I know it's a bit late, but I wanted to let you know that I encountered a similar issue. What it boiled down to was me using the wiring harness for a car that already sends output to a factory amplifier. The amplifier is receiving high-volt input when it should be getting low-volt signal (RCA outs, in effect, or in context of an after-market head unit).
So make some adapters to run your RCA outputs to the harness, and the hissing should go. Sometimes it really isn't grounding, but it's a tricky thing to determine.