2006 Toyota Highlander Noise

My wife has a 2006 Toyota Highlander V6 FWD with approx 80K miles. In the last 4-6 weeks, we have both noticed a noise that seems to be coming from the front of the vehicle while driving. The best I can pinpoint it is on the driver’s side and close to the firewall. The noise first appears when highway driving between 55 and 60 mph. The sound reminds me of an electric motor turning on at a distance, kinda like someone turning on a vacuum cleaner in the next room. It isn’t extremely loud, but it is noticeable. When you let off the gas and drop back down to about 50 mph, the sound is gone until you accelerate above 55 again. Sitting in PARK and bringing the rpm’s up to around 2000 does not reproduce the noise. It only occurs when driving. It hasn’t affected the driving performance of the vehicle, the mpg or anything else that we can tell. It is just a new noise and we’d like to avoid it becoming a bigger problem.