Whenever I start up my '51 ferguson, the bucket is lightning fast and silent, full power. After 10 minutes or so it starts whining, making odd sounds, but still full power. 20 minutes after that, it slows down. 30 minutes after that, I can't even lift a bucket of dirt, it just sits there grinding. Not grinding, but whining. I think the sound is coming from the controls, not from the pump. It's been that way since I picked it up a few months ago, just starting to isolate where the bugs are. Has anyone experienced something like this before?
Here's a video of it progressing. I'd love to get this fixed. https://www.youtube.com/watch?v=9aC4Bl0oofM
If you're sure it's coming from the control valve, then the relief valve leaking would make sense. Look around online at different hydraulic valves and you'll be able to see where the relief valve is pretty quickly. Take it apart and see if there's anything obviously wrong. A pump starving for fluid would make a similar sound, but that would usually be more intermittent, and/or be worse when cold. If there's a suction strainer or filter that would be a good place to check, or if there's any length of rubber hose that could be collapsing would be another possibility. Water in the fluid (milky looking) would also contribute to a pump cavitating. Don't ever go under an attachement that can come down on you. Carry the bucket low to the ground, it puts less stress on everything. If the bucket won't tip back enough so it spills, then lift it up enough to slide the material back in the bucket, then lower it again almost to the ground before moving.
For sure relief valve(s) give us a picture of the control valve, it's not original so it could be anything.