Fan just barely hitting the Shroud maybe?
How is that Drivers side Motor mount? maybe the engine is sagging just enough

 

Mine started doing the same thing. it does not do it in the driveway revving the snot out of it. I only hear it when I take off from a stop in first and moderate throttle. Almost sounds like spark knock. I even tried 92 octane and no improvement. Not sure at all what it is.

 

I know you said it sounds like it's coming from the engine. 'Knock' Are you sure it's not a broken motor mount? In the driveway, Foot on Brake and In Gear, rev engine..what happens?
Have someone look under the hood (From the side of the car) does the engine lift more then it should?

 

I could be completely wrong but my 96 S10 had a similar noise and after I hunted it down I found it to be the distributor rotor was worn and slightly loose. Could be your problem, hope this helps.

 

Brand new mounts. Just install a 65K engine about a year ago.

It sounds like a ping. Not a interference issue.

 

possibly pre-ignition

 

That is what I think. Just not sure why or what to look for.