1997 V6 Accord Engine Noise

My new/used 1997 V6 accord has a low frequency rumble / noise / vibration at around 4000 rpm or 70-80mph. It's not very loud but I think a V6 should run smoother.

The only thing I have noticed is a dent in the exhaust header where it collects to one pipe under the car. Must be from a curb or rock. Could this dent cause an exhaust restriction and noise or is there another common problem with this vehicle.

Thanks