1999 Ford Expedition making noise at 45-50 mph?

I just bought the truck and noticed that when I approach 40-50mph it makes a "brrrr" sound and slightly vibrates. The vibration is subtle. This doesn't affect the trucks performance and there's no loss of power. I'm just curious if anyone else has experienced this with their expedition. The truck has 85,000 original miles and appears to be in good condition overall.

Probably the front wheel bearings. You have hub assemblies that bolt in. They don't come off very easy and an air chisel and good air compressor are necessary.

Check for loose spark plugs! Check the driveshaft and the U-joints. Get the tires balanced. Check the motor and transmission mounts.

Tires out of balance

Have you got big chunky off road tyres?