no engine stated, and this varies by ,, engine.the belts , if new , screech for only 2 reasons1: loose (or oil soaked, watch a guy last weak oil it to stop this)sad.2: or the accessory its driving is seized.id bet the cogged, timing belt will never screech, its idler might.did you inspect all parts as you asssembled the engine.no engine stated, so be hard for the blind to answer with savvy,accurate answers, no?ACCESSORY belts are tension'ed, 3 ways. (3 classes of systems)1; manual tension. means you sit it to spec.2: auto tension'ers 3: some look like auto type and are not.... the book for your engine tells all this.the cam belt as special tension'g steps, did you follow those.we cant guess what you did wrong.nor if you even followed all steps.below is technician loosing the auto tensionerif the tensioner feels weak or bad, it is.my guess, (useless) is that the cam belt is okbut you skipped the accessory belts tension steps.if it didnt screech before.