Car: Newer model Honda Accord
On cold starts, the car vibrates/shakes erratically (but not violently) until the engine temperature gets high enough. When the engine is up to temp, the car runs flawlessly (at least it appears so). During this vibration, the engine's RPMs, however, remain constant and appear very healthy. The vibration is most noticeable when the transmission is in drive, not nearly as much when in neutral. In neutral, you can still hear the car is not producing a smooth, constant sound, but only when it's in drive do you feel what you are hearing.
All of the spark plugs, the engine air filter, and engine oil were recently changed so that is not the culprit. A mechanic suggested it may be a dirty or failing throttle body but my understanding is that if that were the case then the symptoms would always sort of linger, even at operating engine temperatures, and they don't. Or is this not a correct assumption?
Because the RPMs are constant during this vibration, would it not suggest that the problem is not the air and fuel coming in but rather something to do with the exhaust going out? Maybe a clogged catalytic converter that frees up when it's hot?