the i6 knocks. generally lifters or piston slap, just move on.
Diffs, no better time to change the fluid, crack off the cover, drain, install RTV or gasket, fill er up.
Oil.. well, step one will be to find the leak. degrease the motor and keep an eye on it to see where its coming from and report back.
Hard start, well its been neglected so, plugs, a new coil if needed, change the air filter, make sure the gas is good.
Rust, well.... no time like the present time for a good undercoating.