Buzzing Noise From Engine Compartment

Usually I find answers to my questions just by searching the forums.But I can't seem to find an answer to this one.I have a '98 K1500 5.7 Vortec going on 149,000.The only failure I've had so far (knock wood) is a fuel pump. A couple of weeks ago,I noticed ( for lack of a better description) a type of "buzzing" noise coming from somewhere on the top end of the engine. I can't
seem to pinpoint it. I've only noticed it at idle and I've never heard it on a cold engine. It doesn't do it all the time either. The truck runs fine and I've never had a check engine light.All sensors and components are still original.
I can't think of anything else that might be helpful info. If someone has had
the same problem or if someone knows from experience what it might be,I would appreciate a response.

Thanks
Greg

I have basically the same truck, i dont have this problem, but try to eliminate the pvc valve possiblity and then at the top of the engine is the fuel injector system. i can imagine both of these may make buzzing. you could get an engine stethescope or a piece of rubber hose to listen to a specific part of the engine to narrow the search. could it be a fast mechanical sound or is it more of an electrical buzzing?

Thanks for the reply derekbitt. I did replace the pcv.
I really can't say what it exactly sounds like,it could be both of what you suggested.The more I hear it,the sound is like taking a electric motor with a bad bearing (the squeal),lining a box with insulation and putting the motor in it and closing it up.It has that type of muffled sound. I checked all the rotating parts on the engine and they seem fine. I think I will try your rubber hose listening device.

Thanks
Greg

I'm not positive but check the egr valve. These are elecric and operate when the engine has warmed up. Just a suggestion. I would do the stethiscope test to find the area the noise is coming from.Good luck.

Have you checked the cap and rotor, if the distributor bushings get loose, then it lets the rotor touch the contacts in the cap creating a buzzing noise.

Have you checked the cap and rotor, if the distributor bushings get loose, then it lets the rotor touch the contacts in the cap creating a buzzing noise.

I may be wrong,but wouldn't the engine run very rough or sputter once in awhile or worse. I just tuned it in the early spring,and didn't notice any play.
I will check just to be sure.

Thanks
Greg