I just replaced the idler pulley on my 1997 chevy

Replace the serpentine tine belt, I found that some belts will squeal with high mile cars due to the pulleys it self being worn out. Most Auto parts store carry different brands of belt, try replacing the belt there and if it starts to squeal, try another brand. They'll take it back for an exchange, been there done that. If you have a spray bottle, spray the belt and see if the noise goes away, if it does and comes back when the water drys. replace the belt. Good luck and hope this will help.