Just my 2 cents.... Have a 04 BA XR6T with the usual mods making 305rwkw. 2 months after fixing a cronic overboost situation, it developed a tick similar to what you have described, would have sworn it was a lash adjuster also (have been a mechanic for 20 years btw). Anyway after listening carefully with a stethoscope, both inside the rocker cover (through the oil filler cap hole) and on the engine block (you can access it between 5 and 6 on the drivers side) diagnosed it as a cracked piston. This was confirmed by pulling each injector plug in turn (the tick almost went away with no 6 disconected). Pulled my engine down today and yep no 6 piston had fried the crown on inlet valve side. Hope this helps if you havent sorted it yet.
