In reply to ByEek:
Sounds like it might be the callipers like you say. If, when you apply braking pressure to the rim, the pad can jump on and off the rim, using the give in the arm pivots as that little space to jump in and out of, then this might be the source of your squeak. Problem with this is that if there is no play at all then your brakes wont open and close smoothly and will stick - you want play in some parts, and precision snugness in others.
Normally there is play in one of two places -
1. Play in the arm itself. This is when there is play between the break arm itself and the pivoting deep metal ring fitted within the arm that slips over the fork bosses. This is solved by stripping down the arm completely, servicing it and refitting it. If it doesn't work, replace the whole brake.
2. Between the 'pivoting deep metal ring' and the fork boss itself. This is probably a little harder to suss out. make sure that the inner ring and the boss are both completely clean and that the pivot bolt is greased and fully fitted within the boss. It should be sitting snuggly on top of the deep metal ring, pushing it against the small metal plate at the base of the boss with the three spring holes in it . an additional washer could solve things, but it all depends on the original build quality of both the forks and the callipers.
Depending on you available budget, I'd take it to the local bike shop and ask them to fix the noise. New callipers should be stomachable, but if they suggest new forks, stick with the squeak!