Why would a 1990 Pontiac Tempest 2.2 rattle severely at a certain RPM between 45 and 60 kph but run smoothy at all other speeds?

Either your tires are grossly out of balance or you need new shocks. I seen cars going down the road w/ a tire bouncing up and down almost off the ground = Shocks. Get on a four lane road and have a buddy drive along either side to see which wheel is at fault.