My 6.2 has a ticking noise after a cold start ,ever since one glow plug decided to break in half in one cylinder on the drivers side--it goes away after several minutes --I think maybe a push rod or rocker arm got tweaked when it crunched up the glow plug and eventually spit it out the exhaust..
These engines have shaft mounted rockers and no adjustment,and crappy nylon "retainers" to keep the rockers in place,much like old Caddy 500's had..so dont waste your time taking off the valve covers in hopes of being able to adjust the lifters..they are non-adjustable..
I noticed the noise on mine is much more noticeable since switching back to 15W-40 after using thinner oil...I think it takes longer for 15W-40 to get to the lifters and thin out enough to silence them..
Leaking exhaust manifolds can mimic a noisy lifter as noted too--I think you may need to lift the motor off its mounts a few inches to get the exhaust manifolds off if thats the case--the bolts are almost directly in line with the frame rails..
