My car makes a hissing/squealing sound when I press on the accelerator. on 1998 Toyota Avalon

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I just changed my alternator and serpentine belt thinking that it was the problem, but the sound is still there. I' am also in the process of changing the power steering belt in hopes that it will alleviate the problem. I was also told that the sound could be coming from the brake sensor being to close to the brake rotors. This puzzles me because the sound only occurs when I' am accelerating. This sound is frustrating me, because it is becoming to expensive to alleviate the problem. I pray that someone can provide me with answers to fix what is making the noise on my car.
(2) Answers
Noises are often the toughest things to pinpoint and repair, because the sounds that an engine makes are so mixed and echo all over. what works best for me is to get a stethoscope, like a Doctor uses and to probe around the engine. It sounds like it may also be a vacuum leak. I would check for cracked or split hoses. Toyota is famous for splitting the air intake hose between air filter and the throttle body. Also check out any Codes if you have a Check Engine Light, that is also helpful. Good Luck!