'90 745TI (141,000 miles)$1,100 - documented timing belt change2,500 miles ago.

Just purchased this brick & it's been a fine ride so far. (1 month)
I've replaced all vacum lines,changed oil to Mobil1 syn 5w30,changed oil filter,(did this without raising the car if you can believe it!)adjusted downshift cable,(this really made shifting alot better/it was high revving)switched to cone type air filter w/ home-maid heat shield,(removed air intake box)cleaned all intake hoses & components,added strut tower brace(home-made version/made a huge difference in turns)

After all this I'm still trying to quiet the knocking sound; Its mainly during idle and while cold. & sometimes i don't notice it at all.I keep 92oct. in the tank.
It seems every pre '95 volvo motor i here running has this "knock".mine is just a little louder sometimes.
Is this someting I could cure with......
a new knock sensor?
a valve clearance adjustment?
different oil?
or did i get ripped?
I know a more extensive tune-up couldn't hurt. I'm just worried about starting down the wrong path on this particular problem.
The car runs great & has plenty of power.I can't believe it's a four banger!
where should i start?
any input is appreciated.