Weird wobbling?

When I put my key fob in, and I hold down on the brake and start the car with my foot still on the brake.. my car trembles lightly, a little bit almost like a tumbling feeling until I put it into drive and then it stops it doesn't do this all the time but it does this once in a while. can anybody explain to me exactly what this is? It's a '11 Z4 30i
 
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