Volvo Front-end noise

I have a 2002 Volvo S80 with 131K. When driving on the highway at between 60-75 mph a whistle, similar to a distant train whistle, starts somewhat intermittantly from the area of the left front. It normally doesn’t start until I’ve been driving for a while, maybe around 30 miles. When I touch the brakes, and sometimes when I turn the wheel slightly to the right the sound stops, but it will start again after varing periods of time.

The clue here is that the use of the brake stops the noise. You likely have a small vacuum leak from your power brake booster. I would suggest that you have it replaced before that small leak becomes bigger and leaves you without power assist for your brakes.