motor ticking noise at idle only

My 1999 LT 4x4 has a ticking sound from the motor at an idle. Then goes away when you accelerate in park or drive. Noise is only heard with no load on motor. Any ideas? Desperate.

try to narrow down where it is ticking. GM trucks are nitorious for back serpentine belt tensioners. These will tick like made at low rpm if the bearing is shot. Mine does that and i am geting a new one asap. Get a GM one also cause the aftermarket ones really dont last..... like mine hasnt.

If its not the tensioner or idler pulley, or the alternator try a simple test. With the blazer running reach in and feel the PCV metal tube right above the valve cover and see if its vibrating (ticking) - you will feel if and wiggle the tube around - could be a vac leak requiring new rubber or the grommet. Sounds strange but my 99 did that. Caused originally by bad motor mount allowing the engine to lift up and all the cables pushed on the PCV tube.

This may sound stupid, but my 99 LS does it. There was a chirp or a tick that I honned in on. I tracked it down to the AC Compressor. Here is why.

If you have your temperature set to cold inside the cabin, it turns on the AC Compressor for some reason instead of letting cool air just get pulled in.