I just got my car on Monday, and the strangest thing is happening - if it has been parked for any length of time, when I switch from Reverse to drive, within the first 4 seconds I hear a "thump" and feel a vibration in the gas pedal - it is very brief, but happens every time. Once the car is going, I cannot get it to happen again - I have to let it sit for about half an hour or so - and otherwise the car drives very well. Is this *normal*?!

Car: 2003 Accord V6 Ex Sedan
Miles: 230