The 351M in my 1977 F-150 has a slight knock or vibration when it is warmed up at speeds of about 40 mph and above. The engine runs good, doesn't smoke, and doesn't use very much oil. The oil prressure gauge shows about 20 psi warm at idle using 10W30 oil. The pickup doesn't see much highway use. I mainly use it around the ranch off road. I've noticed this condition for about a year. I would appreciate your input.