Also might need to have you look into the throttle body bore - remove the snorkel and see if there's debris plugging the bore on the throttle (presence of a dark sludge ring of oil and soot around the plate) - the "idle vibration" can be from both a bad mount absorbing the shifted load and passing into the car - and/or the throttle body itself needs some cleaning to help with the adjustment changes it needs to make at idle to handle the AC being on and still keep the engine idle set correctly - if it can not move the plate easily - it can make the engine seem to sag or shake at idle because the throttle can't make that small adjustment to trim the idle when the AC is on.
This also occurs on really hot days when fuel boiling is possible - the injectors work, but the fuel itself vaporizes too quickly before it makes it to the cylinder - it weakens the charge (
Usually from too much Ethanol) - this usually goes away after some start/run/stop drive cycles to fix fuel trims. But if it's been really hot, that and a plugged air filter or a vacuum leak (cracks in the line or unseated or loosely fitted hose) in one of the hoses or the fittings can be a tell tale sign it needs a little more TLC.
IF this occurs all the time and more and more noise, then follow
@SyntheticAtmosphere suggestion and look into replacing motor mounts - as to which one? That requires a visual observation as someone needs turn the AC on and Off - and you see which one of those mounts moves the most and may be the one failing causing that shake - press (carefully - hot moving parts are nearby!) on the motor by that mount and see if that shake stops - then the mount it's (your pressure applied) is the closest to - usually means that mount needs replacement. IF you replace one and helps - it also means you'll be replacing others pretty soon - might need to make a day of it to fix this and replace all those mounts.
Loose or broken mounts can also affect your steering by causing torque steer - your having to correct the drift during acceleration and deceleration gets worse with time.
Some use a block of wood to support the oil pan and lift it up slightly and gently with a floor jack to see if the shake can be eased - it can also help change the noise the running motor makes and you can find the bad mount by the rattle it makes from the floating you put the motor in at rest - makes the shake more noticeable on the bad mount.
Don't just look for "band mounts" but loose hardware and broken clips - like a bad clip on the fan shroud or loose cable ties.
