In regards to #1, check the ground cable coming off the negative battery terminal. Could be loose or dirty. Also check the end of the ground cable where it attaches to the body.
Whining speakers often indicate a bad ground.
This can be intermittent.

As for #3, noise from rear at speed could be the driveshaft support bearing, rear bearings, or several other things. Tough to isolate from description.

Knocking while accelerating or shifting is usually the driveshaft flex joints or the transmission mounts.
These last 2 are not particularly expensive to address.

Welcome to the neighborhood, and best of luck.

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1986 300E 5-Speed 240k mi.