"Screeching sounds" aren't good. The ac compressor clutch could be slipping but I don't think it should be engaged when starting. Check the accessory belt and camm belt tensioners to see if the bearings are bad. The front engine mount under the crank pulley could be bad and the metal pieces could be rubbing. Check all the accessory belt pulleys. I just discovered my son was sold a belt that was too long for his S10 pickup. It was allowing the crank pulley and tensioner pulley to rub together. Sounded like a major engine mechanical problem. If it only happens when starting it may be the starter motor but I haven't seen many postings of starter problems. Did the mechanic remove it and maybe not tighten the mounting bolts enough?