I had an idea about this a few days ago, and I decided to make a thread of it. It's really simple:

Different type of ambient noise with different TARDISses.

But Yblocks, how about custom TARDISses, and the entire fact that interiors are structures, making it practically impossible?

Wrong. You see my idea was: make the roundels and/or the console blocks (rotor, lever, etc.) Make the appropriate noise.

And how about mixed blocks and/or custom TARDISses?

That is a suggestion I have to thank Doe_Killer and dhi_awesome for. My other idea I got from them is: change it with either a "Hum block (yes, will get a different name maybe)" with GUI or with the TARDIS monitor. 

But how does the TARDIS work with the hum in larger TARDISses?

You can set which blocks are your TARDIS, and make those hum with the chosen hum. Ofcourse, this makes griefing much easier, so TARDISses must be further apart, which in its turn helps against TARDIS bridging.

N.B.: This is a suggestion, and I am open for comments, so put those down below. Thank you very much already!

The most popular I've seen is TARDISes; but remember, it's an acronym, so TARDI would stand for Time And Relative Dimensions In.

As for the idea itself, I don't want to sound like I'm picking on you, but it sounds very impractical.

What if a player wanted to combine blocks from different eras, which would produce different overlapping hums? Also, every single block emitting a sound is not a good idea. Finally, what if you wanted to use classic blocks but have a modern hum?

A far better idea would be to have a 'Hum Block' that can be placed near an interior which would make the hum sound over a given radius from the block. A GUI could come up on right clicking the block, allowing the hum sound to be changed to the desired one.

Just before I get into anything, there is already a hum, although it is so damn quiet and it is the Toyota (Has been there since the 50th Anniversary update iirc, been getting quieter since).

Having a way to change it to new ones would be really cool, but having it go from certain blocks is quite silly, since you can use multiple TARDISes blocks in one TARDIS, and not even use the blocks in interiors.

Having it be a feature changeable through the scanner could be neat, or the block idea Doe said.

 Good idea, will add it.

Added it!

There's already a hum in the mod.

use resource packs

 That is the toyota hum, which barely makes sound. And when it does, it scares the living daylights out of me. I prefer other hums above the toyota one and it also makes other TARDISses more realistic.