Squeaking under the hood

Hello all and this is my very first car a 96 Chevy Blazer with like 116xxxx miles and I have run into a problem, there is some type of squeaking noise coming from under the hood. Now my mechanic told me it was the idler pulley and I got that changed and that was not it either, then he said some kind of suspesion part or something and I got that changed too but it still makes the squeaking sound whenever I press on the gas and it will also make it when I am at a stop but it is not so loud. Some other guy metioned something about maybe it could be the water pump please tell me what you think because I want the squeaking to stop. I should also mention before this noise started I also had to have the alternator replaced.