Brakes squeal even when I'm not push the pedal.

A grinding or rumbling noise can also be symptoms of a wheel bearing or constant velocity joint in a driveshaft. A worn wheel bearing can also cause a 'whistling' sound (as can a worn CV joint).

A grinding noise with brakes is either worn pads causing metal to metal contact on the brake disk/rotor, or the brake backing plate catching on a spinning rotor ... or a loose/missing anti squeal brake shim (they stop the pads from rattling and vibrating).

However, as you say a mechanic has looked at the brakes and can't find anything wrong .. I think I would begin to suspect a wheel bearing or constant velocity joint on the driveshaft (front wheel drive cars).

sqealing usually drive belts , try spraying each belt seperately and lightly with WD 40 when squealing, whatever one stops sqealing get a new belt fitted

it is the belt it is either to tight or not tight enough

I would check your brake pads to make sure they are good

Hi , Sounds like you need new pads on the front. That squealing is a indicator that th brakes need to be replaced. Also you need to check the computer codes for your brakes system. Go to a repair shop for that. Good luck.

its more than likely the alternator drive belt needs re tensioning if its got aircon that belt may cause the same problem just get the drive belts checked and re tensioned

I am sure you have a defective brake Master Cylinder, the symptoms all fit. That is about all that causes a low brake or sinking pedal.

this could just be "sticky pads" or there worn right down to there limit
you still need to strip the brakes out to check them out,,if there sticky put some copper grease on the back of the brake pads,,not on the brake pads!!!! only the backs fo them this stops then sticking and also from squealing,,

If the back brakes are drum brakes then it`s just water in the rear drum.
The hissing noise would be the hot brake drums boiling the water.

Let me know what you figure. I have changed alternator , water pump and belt on my daughters 2002 Spectra. . Still squeals. Currently assessing battery condition.