You can try Sea Foam, diesel fuel, a retail engine flush, maybe a little gasoline, kerosene, any of those are a solvent that help dissolve sludge, varnish, carbon. Add it to the oil and keep the RPM low. I have used an engine flush from the national chain FLAPS, it has worked on noisy lifters in engines with poor maintenance. If you are religious about oil changes and the oil pressure is normal, you may have issues that an additive can't fix.
