1994 Ford Ranger Engine noise

I just bought this Ranger on a tight budget. Was told it had a transmission issue but that's not what concerns me. When I first got it home, it would rattle/ knock on startup for about 1.5 seconds then smooth out. The engine idles very smoothly and quietly. The engine doesn't struggle to start and the oil pressure gauge reads over half way very quickly. When I revved the engine to 1500-2000 rpm and let off the accelerator, the knocking would happen again for a second just as the engine was starting to slow back down. The oil was very black and seemed thin. I decided to change the oil the next day. Before I did that I noticed very poor connections at both the battery cables. I took them apart, they were very crusty on the ends, cleaned them and put them back together. Started the truck, got the same startup knocking but the higher rpm knocking was gone. It was replaced by a much quieter raspy noise at about 1500rpm.
Changed oil, replaced alternator (old one tested bad). Started truck, the startup noise is entirely gone, but the raspy whirring noise is still there. I put a socket extension (held to my ear) on the driver side exhaust manifold and it sounds like a dry bearing but but doesn't start until about 1500 rpm and continues as the Rpms rise. It doesn't sound like the front of the engine but in the middle or back and down low. But hard to pinpoint. It kind of sounds like when an engine is revved very high and you hear that raspy sound.
So, I don't want it to be bad engine bearings, which I've never heard bad ones personally. I fear it is, but can't explain why the noise changed when I cleaned up the bad electrical connections. Can the transmission (a4ld) make these sounds in Park?
I thank you in advance for any insight you can provide.