I think a loose rod bearing would give a deep sound. I had a ticking when the alternator fan blades were barely striking the bracket. Nothing a pry bar couldn't fix. Combustion sounds can sound like tapping. If in CA, try filling w/ renewable diesel (Propel or Union 76), as my cars idle noticeably smoother than on fossil. In other states try B20 in summer and/or pour in a quart of veg oil.