Whoopin noise @ 27-28 MPH

Ok this is strange. I hear a whooping noise when driving at 27-28 MPH. The engine can be running or on the drive battery. I have the ScanGauge II and the RMP indicating engine running or 0 rmp with this noise. Cruising on a flat road and hold it at 27-28 MPH or set the cruise control to 27 or 28, it doesn't matter, after a short period I hear a whooping noise that gets slightly louder if you keep the speed constant. The noise reminds me of the chirping noise that the brakes do when you push the brake pedal past the point to normally stop, such as a hard quick stomp on the brake. I was told that noise was a brake pump noise and was normal. Only happens if you push hard past the normal point needed to cause braking. This is a similar noise.
Point is this new whooping noise happens running on engine or just battery. My guess is the transmission or inverter noise. The transmission was refilled at 100, 000 miles as was the coolant in the engine and inverter.
It's not loud enough to be heard over the radio or the air conditioner fans, but driving city streets at 27-28 causes this noise always. I know, the quick solution is not drive at 27-28 mph, but something is wrong, as this is a new noise that appeared around 100,000 miles. Again Prius 2006, 119, 000 miles and no problems except the start battery, the little one was replaced. New tires that are quiet and a softer ride, so its not the tires.
Any ideas.

 

It pulsates the sound, in pulses, continuous pulses back to back. Like a whoop whoop whoop.

 

Not the windows or the tires. It appears to make the noise directly in front of the steering wheel from the engine compartment. Put a tape recorder on the inverter and went driving to reproduce the noise. I pick it up but can't figure which area its coming from. Not a wheel bearing which would be the most likely to make that type noise, if the prius has wheel bearings or something in sync with the tires might make the noise, like a jammed stone or nail head. That was the reason to get new tires and have them look at the wheels. But its under the hood and the engine can totally shut down so it should not be engine noise, just battery to the electric motors and its the same quality noise engine running or off. Only at 27-28 MPH on a level road.