What he heard wasn't knocking, but ticking. Higher pitched than a knock, but as you probably figured out, he wasn't real familiar with car terminology.
So he blindly called it knocking.
What the noise is caused from are the lifters. They need LOTS OF OIL to stay cool and to cushion the blow of the cam(s).
It was from his oil level getting low, not because of the sending unit. Well, the sending unit was leaking so yea..... it was "technically" caused by the sending unit.
My wife isn't very religious about checking her oil, and with 190K miles on the engine, she NEEDS TO CHECK IT.
One day I drove it to work and about half way there I had rolled down the window to catch a breeze. With the window down I could hear those poor little lifters getting the crap beat off of them.
I checked the oil level and sure as hell, it was 2 qts. low. Once I added the oil the noise stopped.