I'm new to this site so excuse my future screw ups. I have a 77 silverado I recently bought from my parents. It has sat for the last 2 years until a couple weeks ago. Engine is a gm crate 350, and the trans is a t350 with 3/4 ton running gear. I noticed after driving it a few times that it makes a whining/humming sound after I hit about 20mph. I can't pinpoint where it's coming from but it sounds like maybe the trans or tcase. Both are full of fluid and seem to work right. Any ideas on how I can diagnose this issue? Any advice would be appreciated.