1.if i dont push in the emergency gear release handle, after i started the engine it's starting a weird "knocking sound"..
has anyone come across with this ...phenomenon.. (i admit if i writed it wrong)
Yeah. I believe it's because hydraulic pump is "on" whenever engine is running. Pulling the pressure release doesn't stop the pump.
It might be that to preserve CPU power (and thus provide better frame rates) the pressure release is timered to release pressure at some time interval (talking about less than second, so that the needle in the pressure meter doesn't twitch). This could create extremely small pressure which is released before it reaches to any mentionable level... but the fairing doors might react at anything non-zero.
ALTERNATIVELY (and I think this is a more likely explanation considering how they've modeled other pump-type components), the engine driven hydraulic pump is coded to deliver pressure in jolts because real pumps tend to function that way too, whether they have a reciprocating piston or a wankel-type rotary piston, etc. So it's also possible that the pump delivers jolts of pressure while the hydraulic bleed is constantly open. Since the opening of bleed valve has a limited diameter (which limits rate of discharge), the pump will deliver small variations of pressure in the hydraulic system even with pressure release on. Latter would mean the simulation actually imitates the real world. Latter could also also, even in real world, cause the fairing doors to resonate... the question just is if it's going to make a sound. A sound to occur requires the fairing doors to hit the open limit (there's no lock but there's a limit to how much it can open is present) AND it requires that it takes very little pressure to start raise the fairing door in order to drop it back. But not too little, as it has to manage to hit the limit before it receive it's next jolt of hydraulic pressure attempting to raise it. So were talking about a very small margin... but that doesn't mean the fairing door weren't in this margin (at least in Accusim if not in real world Mustangs).
Even if you were to get the fairing doors to vibrate against the lower limit, it would hardly be audible over the engine noise as to make a loud bang, it has to come down with speed. If it just vibrates down there it'd (in real world) settle againt the lower limit at a slow speed. In Accusim, the noise volume is probably fixed, allowing it to bang unrealistically.
So yeah, I'd say it's a bug, as in "it produces an unrealistic experience" even though the internal modeling might be (aside from adjustable volume) right on the spot.