Take what these guys are saying seriously and don't just assume it is the new carby. You'll never know unless you try everything.
Just because it didn't to it before doesn't mean it is the carby. Something else may have altered.

There's a simlple way to eliminate the carby as the noise source... put the old one back on!!

It may be the simplest option.

thats why I said before that I'm pretty sure that the problem is connected with the carby as I swap them back to see if that noise can be something else but the problem of "rumbling" noise goes away

the "rumbling" noise is like air disturbance I dont know how else you can describe a rumbling noise... like thundering? low bass tone

it is air revereberation, are you also running an exposed air filter on top?
Its alsmost like the noise from the exhaust on valve /timimg overlap after a hard run when you have a cam.... had it on the old motor. it didnt make it run worse, just made me think i was driving a race car some times. its when you come off hard off the throttle after giving it a boot fullof whip ass.
go datsun power go!

Yes that reply is the closest answer i think!

I got ramflo airfilter, foam type
your description is nearly spot on but the noise occurs even on light push on throttle as long as the car is decellerating from 70kms +

the descriptions you guys are giving sounds exactly the same as to the sound my car is making, but i know how mine happens, some guy rewelded the back of the exhaust (rusty i presume) but anyway its on the ever slight angle so that it touches on the back bolt of the leaf spring, if i put rubber in between there no rumble, or if i pull the exhaust to the right of the car, and yes it only happens once revving high and letting the revs come down, its damn weird, the rubbers i put in between keep falling out so she rumbles..i love it

but mine only happens with the weber
so definitely something wrong with the tuning side of the weber or something else relating to the installation

Could it be that the weber is causing the exhaust to give a slightly different exhaust pitch which is creating a vibration?

I've checked the exhaust mounting/rubbers and they're in good nick
also checked the exhaust manifold as well

I'm with Unfamilia on this one.
I think your setup allows you to hear more intake noise.
Is it a bad sound? or are you just making sure it's normal?