Six months ago I took the cylinder head off my 20-year-old Automatic Volvo 740 Estate to investigate a fairly severe vibration in the engine, even when simply idling. I never found a fault but on re-assembly (using new parts as necessary) the vibration had gone. It appears I altered something in the process of dismantling and re-assembly but just what remains a mystery. The car has run OK since then apart from a slight vibration when driving the car hard through the lower gears. Any idea what may be causing that? The car has done 180,000 miles. The cylinders are in particularly good order.