Started noticing a whistling noise in my '03 K3500 yesterday afternoon. The noise seems to be eminating from under the hood while the truck is in motion. I do not hear it while the accelerater is pressed (i.e., I'm moving down the road) but do hear it after I've let up on the accelerator pedal. It appears to commence after a couple of minutes of operation (i.e., it warms up a tad) and it's not the radio.

Had the transmission fluid replaced, an oil change and a new air filter at the local Chevy garage about 5 days ago but that's all I've "done" to the truck lately.

Any one experienced that before?

Thanks, in advance, for any replies. Regards, Jim R.

Make sure that the band clamps are tight on hoses to the aftercooler, and from the aftercooler to the intake manifold.

That's the turbo making that pleasant noise ;)

Mine has a gentle whine/whistle at idle, and always has Jim. It's annoying, but I've read many have it, and it seems to be normal. Reminds me of a bad bearing but *shrug*.

turbo would be my guess

Maybe the new filter is different somehow and is allowing you to hear more of the turbo?
Like posted above check all intake fittings from the filter to the turbo, and from the turbo to intercooler