It is possible that the wear indicators for the brake pads are touching the rotor, indicating worn brake pads. They probably make the noise all the time, but at higher speeds, the pitch changes, and there is more road noise, so the problem is less noticeable. Depending on how much you drive, and traffic situations, etc, the noise should get a lot more more noticeable within the next couple of days. You will then have to replace the brake pads, ASAP. If not, the pads will tear up the rotors, and then they too will have to be replaced.
I had this happen. It was the wheel bearing. It starts off as a faint whine, but over time as the bearing gets worse it becomes a squeal at highway speeds and a squeak at low speeds. If squeaking at low speeds the bearing is on its last legs and total failure is imminent probably causing more expensive damage and repair.
Previas have a history of a whining noise coming from the catalytic converter. There is a bracket inside the converter that when loose or has came apart creates the whining noise during acceleration. It especially is very pronouced during high speeds in the freeway. You might be in for an expensive cat converter replacement.
If you are driving under 30 MPH it probably won't do much damage, but shifting to neutral at highway speeds and you will ruin your engine. If you start skidding on an icy road (probably going 20 MPH or less) shifting to neutral will take you out of the skid and stop your vehicle immediately. Doing this spared me from colliding with another car when I was skidding in snowy weather.
Have your brake pads checked. If the squeaking sound stop when you apply the brakes it may be your brake pad wear indicators rubbing the rotors to warn you to have your brake pads replaced. If you ignore it damage will result when the rotors and metal brake pad backing or rivets rub after the brake pad linings are worn away.
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