Annoying vibration noise - need help for diagnosis

Here I have a 97 galant vr-g.

Vibrates at 40-100 randomly independent to load. Once vibration occurs, it is proportional to rev. Then if I let my foot off gas and immediately put my foot back or even rev up, vibration goes away. Everything seems very odd to me.

Vibration doesnt sound sharp or metal causing. It is a low pitch noise. Its also sounds like a a train going on full speed in a tunnel when I am in the train. Definitely can feel the vibration on the seat and the steering wheel and can hear the vibration noise throughout the whole car.

Has any one experienced the same thing at all? Or any ideas? Is there anyway to diagnose it? I know there are A LOT of possible causes.
I have replaced the rims just for the car's look one month ago so it shouldnt be caused by unbalanced tyres.
I m hoping that its not the transmission. Thanks

wheel bearings, bad axles or unbalanced tires.

Worse comes to worst, broken balance shaft belt if you have I4

I do realize that I am being a joke here when no one can look into my car. Thanks for your ideas. Would heat shield do the same vibration to my car? Would it vibrates the seat and as far as steering wheel? Thanks.

Weird dude, I just posted practically the same question. I just got my transmission pretty much replaced, and that's when the issue started for me. To be thorough, if when you hit the speed that it starts shaking, and you keep your foot at exactly that position on the pedal. will the shaking continue? And if you accelerate harder after it reaches that point, will it stop?

shaking continues for another 10-15km/h then it will fade. Sounds like a harmonic vibration what I mean is only a certain range of frequency of (engine? wheel? ) triggers the vibration.