In level flight I would say nothing else but an increase in ‘windscreen noise’.
As you approach VNE at maximum power it is my experience that the significant give-away is the nose attitude, which is well below the horizon.
The nose on the type I fly stays nicely on the horizon even at Vne, 168 kts, which can be reached in level flight at about 85% or less matched torques. The first indication the pilot gets that he has reached a speed 6 kts below Vne (or hit a gust at normal level cruise speed) is is a woman's loud voice announcing "Airspeed...AIRSPEEEEED" and the knowledge that she has made the DAU log an exceedance "against" the pilot, even if he never reached Vne. Very annoying, a lying female spy in the sky!