Quote:
Originally Posted by humfrz
Let me guess ...... when you turn the A/C ON, the engine is under more of a load and makes that sound ...... when you push in on the clutch, the engine is relieved of turning the shaft in the transmission, thus speeds up just enough that the sound goes away ....... 
Calling @ Ultramaroon for comment ....... although, NOT saying that humfrz is "out to lunch" ......... 
humfrz
PS - Bushhy, I suggest you go down to your local indoor firing range and ask to test fire a .357 or .44 mag revolver (use no ear protection). Then you won't notice half the weird sounds that the FA20 engine makes ......  |
I bet this is just the "neutral rollover" sound.
Because this is a little four-banger, at idle, especially if the engine is doing work like running the AC, it doesn't rotate perfectly smoothly. It could be eliminated by installing a heavier flywheel but who wants that?
With a constant-mesh transmission (all manual transmissions) all the gears are driven all the time. We only pick which pair of gears is engaged to the output shaft.
There's always a little backlash between the rotating components. Just take my word for the fact that it has to exist. Everything just rattles a little in neutral.
Sometimes we can narrow down to which pair of gears is making the most noise by introducing a little bit of drag. While it's making the noise, gently nudge the gearshift into each gate a tiny little bit.
Don't hold it there. Just a quick gentle nudge and listen for a change in sound.
Don't squeeze all the oil out from between the baulk rings in the syncros.
Bushhy, Let us know if that confirms anything.