its not a diesel!... but sure sounds like it

[Dobi]

my engine ('85 22r) is making some sort of noise. It kinda sounds like an old mercedes or volvo diesel...

It doesn't get bad until it gets warmed up. Its especially bad in third gear at low RPM's.

I've heard about manifold leak, piston slap, valve problems or out of alignment

[Dobi]

p.s. It still has descent power, burns about a qt of oil every 2-3 weeks... Temp runs normal also.

Any pointers and how to fix it short of rebuild?

[kjmcdaniel]

Adjust your valves. This can never hurt and it is fairly easy.

If that dosn't fix the noise, you have a more serious problem, as in timing chain, or piston slap. Timing chain is a bitch to replace, but it can be done in your driveway in 2 or 3 hours, but from what you describe I would say piston slap, and there is no fix for this short of a lower end rebuild.