i don't know if it's because i am new to a manual trans, but if i am driving casually (down a residential street, or parking lot or any slower speed) in any gear, and i go over 20hg (by just letting off the gas with clutch all the way out) my car jerks slightly and makes like a whining humming noise. i don't know what it is, and this is my first manual so i am guessing that they make a little more noise than autos do so i may be normal. it only has 3000 miles on it and i bought it in mid august.

 

I'm pretty sure the whining humming noise is the trans. Almost every manual transmission does this.

Everything your experiencing is normal.

 

The jerk occurs because momentum of the car keeps going while the engine is slowing down. This is called engine braking. Your not used to that because auto trans will always shift up to the highest possible gear for that speed. Not to mention the torque converter adds some more cussion. Manual trans is a direct metal to metal link... Which isn't very soft, and once the clutch is let out it isn't letting go either.

The noise is the gears meshing to the opposite edge of the teeth.
The gears are usually ground at an angle from the axis of rotation and with a bit of a "slice" to it for both sound and strength purposes. They are to a certain extent directional though. Similar to differentials. If you flip an axle and run the diff the opposite direction it will make that noise and eventually overheat the gears. But overheating the gears will only happen under permanent/extended use like that.

That's why you only hear that noise in reverse on auto trans cars. Ever notice that?

Don't worry, I have the same experiences in mine. My 1st month with manual trans too. It is no problem. Watch out for it in 1st though. That damn gear will give ya whiplash.