shaking when first turned on/while warming up

thanks for the suggestion. it ended up not starting today. almost turned over, then stopped, as if i didn't keep the key turned long enough; and when i tried again, there was just a shhhhh sound as i held the key to start (none of the usual click or rumble of the engine trying to turn over, and not the sound of a dead battery).

The "shhh" suggests a possible vacuum leak. However, all this is speculation. If it doesn't start, you may have to tow it somewhere for diagnosis.