Well, running poorly at half tank or below is not a sign the pre-pump is bad, but a sure sign the tiny connector hose on the pre-pump is split or slipping. It occurs when the fuel level reaches the split point and the pump draws air. Of course it does not help you to tell you this $1 hose cost you $400, but I am afraid that is why so many of us do our own repairs. Thje old fuel pump could be tested for flow and amp draw pretty easily but it does have its danger working with the gasoline and it requires the ammeter. Still, it is pretty simple and the cut-off point for a K-jet pump is something like 7.5 amps or below (=good).
On the main pump noise, it is always there and has been in all my cars, 240s and 780 included. I guess after 30 years of hearing it (with two fuel pump replacements, neither of which might have been needed) I have gotten used to it. It bothers some people no end but I just ignore it for decades. Does policy not seem to ever have caused me any breakdowns.