`thunk` noise when shifting

[S2000Strube]

I hear what sounds like a thunk every once and awhile after I shift into first gear. Doesn't seem to happen all the time though. Anyone have any ideas why, is it normal? Car is an 05.

[vortechs2k]

I get the same thing, but mostly when it's cold. I've heard it helps to throw some better tranny fluid in there, but I haven't done that yet. I've got an '04.

[repiv]

This is perfectly normal for most manual transmissions. Here's what's happening:
While you are idling in neutral, the tranny gears and shafts are spinning at whatever the engine rpm is. When you go to put it into gear (especially 1st), you are suddenly bringing all those spinning bits to a STOP! Why? Look at how a drivetrain is put together .................
The engine is connected to the tranny via the clutch. The tranny is connected to the rest of the drivetrain (propshaft, rear diff, CV joints, rear wheel hub splines) via any gear inside the tranny.
If you push in the clutch, the engine gets disconnected from the tranny. If you have the tranny in neutral, the tranny is disconnected from the rest of the drivetrain. Know that for all those gears, cogs, ring and pinion gears, CV joints, etc., there are tiny little spaces between the teeth (or mating surfaces.
Since the tranny is spinning and the rest of the drivetrain is not, when you throw it into gear, the back end of the drivetrain (anchored to the ground) slams the spinning bits in the tranny to a standstill AND the spinning bits of the tranny tries to get the downstream end of the drivetrain to spin. That inertia smashes all the little gaps in the drivetrain together. This is the "thunk" you hear and feel.
How to remedy this? ............................................
Next time you are sitting there in neutral and you wish to engage 1st gear to get going, do one of the following things:
1. Push in the clutch and wait 2 to 3 seconds BEFORE you move the stick into 1st. This slight delay will allow the viscosity of the tranny fluid to bring the spinning parts to a stop.
2. After you push the clutch in, put it into 4th. This will stop the tranny from spinning, but because of the ratio of 4th, the "slamming" effect is almost nearly completely absorbed and you don't hear any thunk. Then you can immediately shift into 1st to get going.
3. Don't throw the shifter into 1st so fast. Move it more slowly so that as the gears mesh, the spinning parts can decelerate less suddenly.

There is nothing wrong with your car. It's the physics of how it's put together and how mechanical things work.
"Better" fluid will only cushion the "thunk". It doesn't change the natural physics of what's going on.