I drive a 2000 4 cyl. 2 wheel drive 5-speed manual. A few days ago I started hearing a high-pitched whine like road noise when I was going 60-70mph. I can let off the gas or push in the clutch and the noise goes away. It's gotten worse in the last couple days, to where I hear it in all gears while accelerating. There's no noise unless the truck is in gear and either accelerating or maintaining speed. My best guess is low gear oil, but thought I'd throw a post on here before I climb underneath with a wrench and a bottle of oil. Any thoughts?