High pitched whistle

[band nerd]

2014 2500 Cummins, stock, 199,500 miles .
Going down the road under normal load I get a slight high-pitched whistling sound . If I turn my head I don’t hear it . It also goes away if I let off the accelerator . it is not speed-related . Turbo and DEF pump were replaced a little over a year ago .
Yeah, I know, just keep my head turned .
Thoughts?

 

[chri5k]

I had something similar on my 2500 with 150K miles. It was not speed dependent but it was engine RPM dependent. In park it would make a faint high pitch sound in the direction of the dash / steering wheel when I got to 1400 RPM or so and go up from there. When I stepped on the brake or turned the wheel it would stop. This lead me to the power steering / brake booster hydraulic system. I set the engine idle control to raise the idle to where it made the noise and probed with a stethoscope. It was strongest at the fittings on the back of the pump. It was not bearing noise. The noise did carry through the lines to the ABS block. I think it is fluid squeezing through a valve somewhere in the pump. I might replace the fluid to see if that changes the pitch to where I can't hear it.

 

[plante2017]

I’ve have a high pitch whistling sound in my truck - i isolated mine to the air conditioner and more specifically to modes of the ac.
1) when the recirculating air is selected (no ambient air entering).
2) when max ac is selected.

these are either or both selections eg if I have recirc on and/or max ac selected I get the whistle. this suggests to me that the box where the air is mixed- when it ought to be sealed from outside air - is leaking a bit and probably drawing a little ambient outside air.

 

[plante2017]

BTW this was probably going on for some time but I couldn’t hear it due to the mud tires on the truck when I bought it. I have since punted the mud tires back to stock tires - the truck is way quieter…except for the whistle.