I used to have an Old English Sheepdog that would nip you in the butt.

Butt I digress. Sounds too subtle to set any fault codes, though it's always worth a look. I would guess something like dirty-ish injectors. When the ECU begins the lean out the fuel mixture at the end of the warmup cycle, if the spray pattern is a bit compromised, the engine won't run quite a smoothly. Try using a good injector cleaner. Or possibly it is just old engine mounts which don't absorb vibration quite as readily as when they were fresh. Seems likely to be an interesting problem to troubleshoot. Most owners don't go after things this small.

- JimY