Diesel Bombers - newbie from new jersey

i have an 03 2500hd with 127,000 miles.engine is still stock with original injectors. bought truck from dad-in-law before he passed. i miss him but i like his truck. at this point in time i have 2 issues that need addressing before any mods are made. first is a lot of ticking noise coming from the valve covers. loudest is on left front. the second is on unusual and occasional jump in coolant temp. both thermostats have been replaced with delco thermostats and functioning. my thoughts were to adjust the rockers and flush the dexcool out of the system and relpace with a quality diesel rated anti-freeze. i have been a technician with the post office for twenty years. i have been trained on the ihc 6.9, gm 6.2, cat 3116 mechanical rack and injector. i was told at the local chevy dealer to be very careful adjusting the valves. they have had techs blow up engines,so he said. i think he was trying to scare me into letting them do the work. any ideas and suggestions would be appreciated. my experience with ecm controlled diesels is very limited to glow plugs on 7.3 fords and routine maintenance on fords. i don't believe the head gasket leaks because i never have to add coolant and the temp could be fine for weeks then jump to 190 for no apparent reason. then drops back down as im watching the guage. my other toys include a 38 foot rv travel trailer and a 2004 harley firefighters road king. i am looking forward to getting to know everybody :c: