Any idea what this noise might be?

[bas157]

Truck is a ’05 4x4 supercab with about 21,500 miles on it. Automatic tranny, manual hubs. Entirely stock truck (except tinted cab lights and windows which certainly wouldn’t cause any noise)

I noticed a noise while driving yesterday and have no idea what it is from so I’m hoping someone on here might know.
The sound occurs after I remove my foot from the accelerator peddle, between the RPM ranges of about 1700 to 2400. Below 1700 I don’t hear it and above 2400 I don’t hear it, not sure if its just because the engine/exhaust drowns out the sound at the higher RPMs
When I step off the accelerator pedal, I only hear the sound for maybe 2 seconds, probably less time really.

It sounds like something is rubbing, like I want to say what a radiator fan would sound like if it where just barely rubbing on the shroud. I only hear the sound for about 6 times though when I get off the pedal, its not a constant sounds, like something is spinning and hitting something, but the radiator fan I think would be making more sound as it should be spinning fast than that.
It is not very loud but is noticeable, just ~6 really quick sounds in rapid succession.

I also don’t hear the sound if I’m in neutral or park and stepping on the accelerator pedal, its only when I’m driving that I hear the sound.

I first noticed it yesterday near the end of my ~35 minute commute to work, on the way home, I heard it for about the first 25 minutes of the ride home, then it stopped making the noise. I heard it again on my way to work this morning. I’ll see if I hear it again on my way home tonight. I just don’t know what it might be.

At first I thought it was turbo fart, not really sure what that is from or what it sounds like on a stock truck, but I have heard of the term and saw a video on youtube a long time ago

There are no warning lights on the dash and the truck seems to be running and driving fine though. Its been cold the past few days(in the 20’s). The block heater comes on a few hours before I head to work, during the day while at work it is not plugged in. I don’t have a code reader at my disposal.

Anyone have any suggestions or ideas on what it might be?

Thanks
Bruce

[RedDye01]

Mine has been making mystery sounds too and its really starting to piss me off. Ive got my eye on the vacuum pump. check it out.