I have a 99 Durango, it's getting old and showing its age and I have been replacing parts. Recently (like the last two months) I have been getting this high pitch, dog whistle like sound when I let off the gas. The sound only last for about 5 mile per hour of deceleration. I have to be going at least 25 mph to replicate this sound and it immediately goes away if I press on the gas, or after the engine or vehicle has slowed. It does it the same way at highway speeds and its the same sound. 55 to 50 etc, etc. no rumble. 200k miles. Vehicle has a new belt, alternator, and Water pump(although I recently noticed I smell antifreeze) that being said I believe it is coming from the right drivers side, at least to the best of my ability I think that's where it's coming from. I wanna say it has something to do with engine speed but the fact I cannot replicate it in park has me thinking it has to be a moving part(like tranny, driveline, wheel bearing, maybe even the front u joints) by the way it is a 4wd but the truck stays in 2 high all the time but I know these other parts move still. I cannot see anything wrong with performance as it runs like a champ still. I have scoured every forum and nobody seems to have quite the same issue as me. I am just wondering what could make this sound? Oh and it seems to be able to be replicated if I stomp on the gas and get the rpms up as it idles down, again I have to be moving. I am not extremely car smart but could this be connected to 'engine braking'? If there is such a thing?

 

The only thing I've ever had whistle on cars is power steering pumps.

 

I guess, I just don't see how it could make that noise whenever I am driving straight, doesn't change when I am turning, and the pump isn't whining, I guess it could be a bearing in the pulley of the pump?

 

Or a vacuum leak.