mtbscoTT

Just took my first extended trip in my 2001 225CQ, 1981 miles from Houston to Clemson, S.C. and back. The ride and trip were great but I noticed for the first time a very rhythmic droning noise in 6th gear with the cruise set at 80mph, a steady surging noise. If I shifted back to 5th, the engine noise was louder of course but the drone and surging went away. What causes this? My passenger and I alternated between music and conversation and it was very noticeable when the stereo was off.