I also having this issue in my 2006 xli, at dat specifice rpm it makes some noise and feeling like grinding in engine, its due to broken intake manifold, as these cars having low quality intake manifold material so by high rpm as mechanics do for checking 7 and above, due to which air also come to manifold fast, and due to that air pressure internally manifold i broken into several pieces, so whn u accelerate these broken particulas vibrate and make dat sound,
Solution is simple as i did, i opened dat manifold by a cng mechanic here in peshawar, he opened the whole system and cleaned, now my car is perfect, one solution is dat to change dat intake manifold, dont know abt price,