The Houston Chronicle reports in its article "Severe vibration problem plagues moon rocket design" that NASA officials stated, "These longitudinal forces may increase the loads experienced by the Ares I during flight, and may exceed allowable loads on various portions of the vehicle and allowable forces on the astronaut crew.”

Ares I is the manned version of the Ares project. Ares I’s first stage consists of a single five-segment reusable solid rocket booster (SRB), much like the twin four-segment SRBs that are used on the space shuttle program. A SRB burns a specially formulated and shaped solid propellant. The SRBs are built by ATK Launch Systems (Brigham City, Utah).

Ares V is the heavy-lift version that will be unmanned, built to ferry heavy cargo, supplies, and spacecraft back and forth between space and Earth. It uses two five-segment SRBs for its first stage.
The problem now occuring with the Ares I is that gas vortices inside of the SRB add pulses of acceleration to the vehicle. And, these vortices happen to equal the natural vibrational frequencies of the motor’s combustion chamber—when present the two frequencies are added together, causing excessive shaking of the SRB and, consequently, of the entire vehicle.  

Engineers studying the problem compare it to the wake that results when a speed boat is traveling through the water here on Earth.