Hi Clarky, I had the same thing and I put it down to the Soundaktor which, as far as I understand, vibrates the windshield to make that artificial engine sound. As default, the Soundaktor is at its strongest in Sport mode which is possibly why you only notice it in that setting.
I toned down the strength of the Soundaktor (to 50% in all modes) using Obdeleven as many others have done and I haven't noticed it since. However, if you do want to keep it at 100%, you might need a trip to the dealer.