Bicycle Mechanics - Rattling fork when front braking

Anyone know what could be the problem?
When I heavily break from riding fast, the
fork will rattle forward and backward.

There are a couple of moving parts in that equation. First is your headset bearing. Does it have play? To check it, Hold your front brake firmly and push the bike back and forth. There should be no "thunk" feeling. If there is, tighten your headset bearing and recheck.
If it persists, check your wheel bearing for play by pushing it side to side with a light force.
If it is not that, then check your brake calipers themselves. The tolerances for this sort of play are a little more open than the other bearings, but it can be them too.
If all of them are where they should be and it still persists, it may just be fork flex too. ;)

My first thought is also that your headset bearings are loose. There's a chance that it's your wheel bearings, too, though. And of course, the front brakes.

The rim may also uneven in width and friction. Pads sometimes melt onto the rim and the deposit has different friction than the are metal. So the brakes will "skip" on the rim when it hits spots with varying friction. Rims may also balloon wider at points where there's been an impact, maybe not enough to see, but if you pull calipers out and measure the thickness at even distances around the time, say.. 2" segments, you may find that a lot of rims are off by more than +/-0.5mm, which leads to juttering brakes.

It could simply be brake chatter. Add a little toe in to the brake pads and see if it goes away.

last but not least, if it is a suspension fork, if it's out of adjustment (loose), that could be causing it.

park tool repair help (http://www.parktool.com/repair/bikemap.asp)

I tried braking while still and pushing the bike- it does
move forward and backward a little as i push it back and forth.
It seems like the fork has a little too much room where it mounts
to the frame