What could be causing idle shaking on a 1990 Audi Coupe Quatro?

Hey guys, I have a 1990 Audi Coupe Quattro and it's been shaking at idle. I replaced the front left motor mount because it looked like it was the one that was vibrating, but after I finished the engine was still shaking. (The engine itself is actually shaking very slightly under the hood) What do you guys think it could be?|||it sounds like a mount if the motor is running fine|||try some lucas fuel injection cleaner,around 5 bucks 2 see if it clears up,but usally it will b a bad wire|||It may need a tuneup, but if this started suddenly it usually is a vacuum hose leaking.|||If it's not a loose engine mount itself, it could very well be a valve. I have an Audi as well that had a shaky idle, service had replaced a faulty valve and the shakes were gone.