I have firestar inveter 8,5kva 6,5kw 100mppt , the fan runs noisy non-stop  and less noisy when it is switched off even when the battrey is full no load and showing no fault what could be the problem?

I played a bit with a 2kva fivestart inverter trolley. That fans sounds worse than a jumping castle. You have 2 fans running in push pull configuration,  which feels totally overkill.

I disconnected 1 fan but the unit needs the fan turning feedback to stay on. If I can find a way to bypass the fan failure trip it would be much more quiet.

I'm contemplating to mount the second fan somewhere inside the eclosure just to circulate. 

I'm going to run a bridge piece from the running fan's puls wire to the non running fan's pin. If it work you can put a thermal switch on the heatsink, which can switch on and off as the temperature varies.