One possibility is that the fly-wheel bolts have come loose. Does the sound change when you push the clutch in?
So you already removed the BS or you are planning to. If it was done improperly you could have left an oiling hole open and that would give you low oil pressure. Are you saying that you lost oil pressure already or is that speculation? If so then you may well have spun a one of your rod or main bearings which could also sound like that.