Sounds like the ball bearings in the deck spindles are worn out. The spindles will most likely need to be rebuilt or replaced. Idler pulley bearings can make noise too. Remove the deck and check for noisy worn out and loose bearings.

Mower: I agree with Ray , sounds like bad bearings . You can remove the Belts and check for Radial and Thrust end play on the spindle shafts . Same with the idler pulley bearing spin Pulley after your belt removal and listen for rough bearing chatter which will become growl (grinding noise) or squeal during Blade High Speed Rotation .

Thank you so much guys. I'm going to check those spindles & bearings and will let you know what I find.

I checked 2 spindles that turns the blades, they sounds OK and I pumped grease into them. The 2 idler pulleys are solid, they don't have the grease valve so I dropped some machine oil on them. I found a pulley next to the blade's spindle on the left side is a little chatter. Could the squeal come from this pulley? The noise is louder when the mower in full speed. Please if you could give me the part number of this pulley? Thanks for your help.

Mower: Well you have validated a squeal rather than a growl . The Growl is normally a low speed very worn out bearing & race . The Squeal although similar can be related to just a "dry bearing situation" on a High Speed
Application. So the answer is Yes ! Replace the bearing asap , to avoid any other subsequent damage . Mownie or Rusty and CG or Tomplum are the Resident Forum Experts on Lawn Tractors , You will have to Supply Manufacturer and Model Numbers for the Mower not the Engine for them to assist .

Note: Raystrom , Sorry I did not give you equal Billing will do so when 2-Stroke Motorcycles are Involved !
Sorry , You know I'am just pulling your leg :)

ewalk: Thank you so much for a quick reply. My riding lawn mower is Sabre, model 1542HS made by John Deer. The pulley that I'm thinking a defect is the medium size on the left side of the deck, there are 3 of them on this side, 1 for turning the blade, 1 small and 1 medium size. I need the part number for the medium size pulley, and if you could please give me the number for the small pulley too, just in case. Mownie or Rusty and CG or Tomplum, please help me on this. Thank you all.