Vibration under the engine from around 3kph to 0
 

so I noticed a vibration under my car (mainly under the engine) everytime Im letting the car to stop in neutral (no brake pedal pressed). the vibration starts when the speed goes down to like 3kph until its 0kph then it goes away when the car is in a complete stop. the vibration is feel like when you slightly let go clutch a bit quick on the 1st gear that it shakes the car. (I suspect the drive shaft but maybe I am wrong).

Sounds normal...
But to be sure, check all your fitments and connections. Make sure there are no leaks in the engine.
Do that first. Odds are if you found a leak or something disconnected, there's your issue

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Are the engine rpms dropping?

Ok, let's start with the unanswered stuff first.
What did you do to your car before it started?
Did you hit something on the road? Deep pot hole? Jumped a curb?

okay so I went under the car to check if there's any leaking parts but everything looks good to me
* and before I noticed that, I passed by a really deep train tracks and feel the heavy bounce of my front and thats what I only remember before noticing that problem
* and right now I can hear a humming sounds (increasing when speeding) under my rear wheel, that's after I changed my rear diff fluid with motul 300

engine rpm is not dropping and seems to be a fine normal idle every starts

okay so I went under the car to check if there's any leaking parts but everything looks good to me
* and before I noticed that, I passed by a really deep train tracks and feel the heavy bounce of my front and thats what I only remember before noticing that problem
* and right now I can hear a humming sounds (increasing when speeding) under my rear wheel, that's after I changed my rear diff fluid with motul 300

Have you checked your tyre pressures?

I've noticed my car doing something similar since I started using the air conditioner this year. This is my first year with the car so it may be standard behavior for these cars with the a/c on. My car does more of an audible rattle rather than a shake, but it sounds similar and under similar conditions. Point is, if this is happening with the a/c on, see if it also happens with the a/c off. For me, realizing the relationship put it out of mind that it might be an actual issue.

I'm hearing it with the a/c off and even if the car is off and I tried to push it forward, I can feel the vibration under the engine bay ( that feels like a gear head that missed its thread to the other gear head)

I think I know you're what you're talking about. Whenever I come to a stop usually at a light. I step on the brake and slow down to maybe 1 or 2 mph and with the brake off, when coasting to a stop, I feel a vibration. It feels like the ABS pulsing. Until I come to a complete stop. It doesn't happen all the time, but pretty often. It's done it since the car was new (has over 130,000 miles now) so I don't consider it a problem but it is a curiosity.

yeah it does happen everytime if I roll from around 3kph-0 slowly without pressing the brake but if I press the brake a little hard to sudden stop then it doesnt feel anything ( maybe because the rotating thing suddenly stop thats why it doesnt feel the vibration)
*Anyways I'll take a note on what you said and thanks for the response 👍🏼

Maybe you have a damaged tire. Do you get a little wobble in your steering wheel?