I have a 2005 silverado with 205,000 miles on it...it develops what sounds like a lifter ticking when going down the highway at highway speeds...when i let up on the accelerator it still ticks and is in relation to the engine rpms...this is not the ticking at start up most people complain about ...but happens when going down the highway and will continue for a few miles and then go away till another day...i have tried some marvels mystery oil and changed my oil but it continues on and off but not what i can call a regular basis...
i have read a few things about the pick up o ring going bad on the oil pump....so any gm mechanics care to weigh in

It's tired.

Broken manifold bolts?

exaust leak!!!!

Could be the purge valve/solenoid located on the left side of the intake manifold near the throttle body.
http://www.rockauto.com/en/moreinfo.php?pk=3785916&cc=1431121&jsn=460

What have you found?

Fan?

I would guess exhaust leak.

Did you check the clock? Clocks making ticking noises

Exhaust leak

Blower fan?

Mine is shy of 200k and on setting number one the blower fan makes a ticking noise. Turn it to 2 for a second then back to 1 and the noise is gone for a while.

Exhaust leak?