I definitely think you should take your FA's concerns into consideration. There have been many instances when airplanes struck things while taxiing on the ground and the pilots never even noticed. 300,000 lbs of airplane striking a small 3000 lb vehicle with a wing that's 100 feet away could go completely unnoticed to the pilots in the cockpit, but the FA's might notice. The same can be said for strong vibrations in the rear of the airplane.