I'm not very literate with mechanical stuff... so I really have no idea what's going on.

Whenever I hit brake, occasionally I hear this sound going "ta ta ta ta ta". It's not the typical smooth squeaking sound. The rate of this ta ta ta sound varies with the speed I'm travelling at. The rate gradually slows down as my car slows down. Anyone have idea what's causing this?

 

Baseball card in your wheel spokes?

If there's fluctuation in the pedal while this is happening it could be warped rotors.

 

it does sound like metal hitting something.

 

a nail in your tire?

 

definitely not from tires... from inside of the car

 

Good guess, this was an accurate diagnosis once online in the Toronto forum.

 

one possibility is the differential. i started hearing this before, same as you describe...about a few weeks or so later it got really loud. diff was pretty much shot so had to get a new one.

 

but he said this only happens when he applies the brakes? (not on decel / coasting?)

OP: perhaps you can isolate some of the possibilities by seeing if this also happens when you're slowing down w/o touching the brakes -- if this happens too when you don't touch the brakes then it shouldn't be brake related (which also means it could be drivetrain related).

 

sounds only when braking. Once I started braking, i hear little sound of metal rubbing something going kee kee kee kee. It seems like at a certain point of tires' rotation the sound comes out. When the car slows down, the rate slows down too.

 

if you're sure it's not a nail in your tire...

then you need to inspect your rotors and brake pads.