I have had a buzzing sound coming from under my hood for the past 2 weeks. it is so loud you can have to talk louder in the cab to hold a conversation. when the engine rpm increases the pitch of the buzz goes up. I dont have to be driving down the road to produce the noise the engine just needs to be running.

I thought it was an alternator bearing so I replaced the alternator...noise still present.

the noise isnt the turbo whine because that is even higher pitched this noise sounds more like timing gears on a street rod.

any help would be great.

Take the belt off and if it goes away most likely its the power steering pump...

I already checked the PS pump

Do you have some increased vibration that goes along with the buzzing or the droning?

I am going to take some acceleration vibration readings tonight to see if I can get a frequency of the vibration. I am going to put timing marks (using white-out) on my pullies and use a strobe set at the frequency I get from the vibration readings to find the bad actor.

if is not anything that is associated with engine speed or up around 10,000 - 20,000cycles per minute then I will know its probably the turbo.

The stepper motor for the turbo makes a "buzzing" sound when it's running. I flashed a tune in that wasn't quite right and it buzzed real loud. I thought I had an electrical problem.

all the vibration seems to be coming from the alternator at a high frequncy (about 120,000 to 150,000 cycles per minute which is about 2500 Hz)

I just replaced the alternator with a reman delphi unit so I am not sure what could be causing the alternator to buzz

The sound was an idler pulley; the one next to the tensioner. if anyone ever hears a sound like timing gears or high freq. gear mesh check the idler pulleys first!