Toyota Repair: 93 Prism 200k, 5spd, cold-start idle vibration, motel bed, motor mounts

Question
Recently bought car for my daughter.  Solved similar problem w/wife's 94 corolla wgn by replacing frnt, rear, and right mounts.  Replaced all the Prism mounts, improved but got worse with new left (Anchor product). Eventually went back to orig left and orig right- that seemed to do the trick - smooth cold start this afternoon (80 F)and wife then drove it 30 mi and said it was great.  But, then back to cold-strt vibes when moved it this eve (60 F).  Vibration @ 1000-1100 rpm is v.low audible,like vibrating motel bed, maybe 30-50 hz, makes loose plastic door lock buzz. Car has nearly new gapped plugs, wires, & new rotor, cap, filters, timing belt, etc.  Are the mounts that finicky? Could it be something like a sticking EGR? Aging sensor? (no chk engine light) Gummed throttle body?

Answer
The only reason that you get this kind of vibration is because of loss of insulation between engine/transmission and the reat of the car body. A lot of times this can be caused by motor mounts or a part of the exhaust system making contact with the body, this type of vibration usually changes with engine speed, it can also be caused by an idle that is too low, evey engine has a certain amount of vibration, that's normal, in most cases engine insulators (motor mounts) absorb most of it so it's not noticeable to the occupants.