Varnish build up in the engine can cause one or more hydraulic lash adjusters to stick and cause a ticking noise in the engine. The noisy hydraulic lifter will need to be located and cleaned to stop the noise.Google+
Car Problem Reports
Isuzu Rodeo Varnish buildup in engine may cause ticking noise
Isuzu Rodeo Problem
Engine Affected: 3.2L V6
Visitor, 1995 Isuzu Rodeo, 3.2L V6, 140,000 mi
I am now at 201K miles and I use Lucas Oil Treatment with every oil change, after 30 mins of driving the ticking stops and you are good for about 3500 miles and then the ticking starts again, this is normal for oil breakdown)
I have tried the following treatments to stop the tick; STP Oil treatment, Marvel Mystery oil, several PPTF or teflon based engine treatments only Lucas Oil reinstatement works to keep my sewing machine engine quiet. IT much cheaper than a lifter.