Last night I'm driving on a road I travel frequently(smooth/new/flat). My speed is approx 120 km/hr and my MO starts to rumble. At first I thought it was a flat tire. Upon checking the tire pressure gauges and visible check I noted no problem. I continued on and at 110 km/hr the rumble returned(disappeared at 90km/hr). To describe it better, it felt like there was backpressure on the engine, similar to an engine brake on a semi.
This is the second occassion in the last 2 weeks. The truck has 7400 km's on it.
Any ideas???

This is the second occassion in the last 2 weeks. The truck has 7400 km's on it.
Any ideas???