Are you sure its not the bonnet rattling? I had this issue on my car I recently bought. Usually when letting out the clutch and the revs got to low, you couldn't feel the car or engine shake but the bonnet certainly did. Turns out there are two rubber stops that the bonnet sits on, open the bonnet and turns these counter-clockwise and they will lift up, tightening up the bonnet. Might be worth a try?

Ollie