There's something wrong w/ my car...

yes yes yall

check the rotors to make sure they are not grooved, glazed, and or warped. Grooving its caused by over worn pads basically eating at the rotor surface. Glazing comes from replacing the pads with out mating(sanding 80 grit will do ) the surface of the rotor and pads. By sanding both surfaces it will create a fresh contact area free of old brake dust deposits which build up on the rotor to create a mirror like finish that decreases stopping power, and causes vibrations unders braking due to the pads loosing friction. Warped rotors will be very hard to ID. best to let your local brake shop check them out, plus if they are warped they have the proper tools to resurface them back to a new finish. The warp on the surface of the rotor causes vibration due to the uneven contact area between the pads and rotor. Then theres the bad cause which could be anything from a busted CV joint to God forbid a tweaked frame due to a previous collision. If not sure that your alti has been abused by its previous owner, you can get a vehicle report from the dealer free of cost.

check and see if the caliper bolts are correctly tightened or if your missing an anti-rattle clip these will also give you those sounds your hearing

We seem to have the same problem with our altima its a 98.. and we just had it inspected and they are saying it is the upper control arm bushings and that is cause the noise when we brake. so might want to check that out also.