Car shakes hard at idle

[IDriveFords]

Today as I was coming home I noticed at traffic lights my car is vibrating pretty hard at idle. My first notion was that the motor mounts are shot since the car has over 150k on it and they've never been changed. The problem with this theory is that when I got home and played with it for awhile it quit and the car seems normal now (the motor had alot of play at first). It was also making a faint clanking noise that sounded like it may have been coming from the exhaust, but that went away when it stopped acting up and I wasn't able to pinpoint it. I have some nismo motor mounts a friend gave me ages ago and I just never bothered to put them in becuz I planned on using their life for an SR swap. The problem disappeared and the car sits at idle like normal now. Any ideas/suggestions as to what it might be?