When I am moderately accelerating around 40-45 mph and the RPMs are slightly over 2,000... if I suddenly take my foot off the gas allowing the RPMs to fall, as the tach passes 2,000 and continues to drop I hear a whistling noise, almost like a tea pot or a dog whistle.

I did some searching of past posts and while I have found a lot of posts describing whistling noises that sound just like this, I haven't found a single one in which the noise occured when mine does. I do not hear the noise as the RPMs increase, nor do I hear the noise if I keep my foot on the gas and continue accelerating up to cruising speed. I also do not hear the noise in nuetral, nor when I am in Park and rev the engine up through the RPM range.

Now when I take my foot off the gas after accelerating, this should cause the manifold vacuum to shoot up. I am wondering if there could be some sort of sensor or actuator like PCV or something else that could be whistling as the jump in vacuum pulls gases through it... I do have a K&N filter installed in the factory intake, but if that was the cause I would think I'd hear the whistling noise with higher RPMs under acceleration, not during coasting.

If anyone has any other suggestions, please let me know. This problem is driving me crazy, as you can see that I am online here at 1:15 AM trying to figure it out. Could it be vaccum/air intake related, or should I be suspecting somethign belt-drive like an alternator bearing getting ready to go out? Could it be trannsmission related?? Anything other ideas?

 

Dont know... A friend of mines 7.3 powerstroke has an air intake, exhaust and tuner on it. As soon as you take your foot off the gas it almost exhales, like a whoosh.I hear its normal on diesels, dont know about gas though. Sorry I couldnt help...

 

Your vac pressure doesn't increase that much when you just rev the engine and let off the gas.

but when you are driving the engine is under load and therefore the pressure is much higher on decel

so I am guessing that you have a very small leak. I would check;

brake boost line
all pcv lines
all things connected/screwed/attached to your k&n

also if it is always at the same speed it could be something on the body of the truck that hits resonation (sp?) at that speed and perfect rpm/engine vibration during decel

good luck-

 

The automatic guys don't seem to get this as much as the manual guys, but I've confronted this problem several times, first time at 2700 rpm the second at 2400 rpm. After advise from a member on this board, I blocked the heater hose with a pair of vise grips and the noise went away... Flow restrictor in the heater hose is the culprit.