Anyone got any ideas what could be causing a metallic squeal from the rear of the engine around the valve cover area. Does not do it on tickover, only when accelerating fairly hard but goes away when in 4th gear at 30mph. Will not come back unless I sit in heavy stop start traffic. Plenty of lubrication on camshaft, air blown through oil passages and clear. Turbo swapped but still the same, engine flushed to be on safe side - no change.