Q: What would cause a high pitch ring at high speeds in my 2002 Chevrolet Silverado

When I first noticed the high pitch ringing, it would start usually at about 70-75 mph. But recently in the last few months the riniging sometimes starts at 50-55. I thought about the rear end but the sounds seems to be coming from the engine bay. My only other guess would be the transmission. If you could help me figure the problem out I'd really appriciate it.

My car has 161000 miles.
My car has a manual transmission.