It would help the guessing/diagnosis to know what parts have already been replaced. Broken valve train parts would be the best you can hope for. If the engine has been making bearing noises, permanent damage is already done, regardless if it turns out to be an oil pressure problem that they corrected. Any bearing damage will lower the life expectancy of the engine considerably and you should force them to replace your engine. Can the noise be better described as a "rapping" noise?