Question 2002 Tundra 4x4 Engine Howl

[17acres]

I just went over 100k on my Tundra. I have had zero problems with the truck until recently. When I accelerate there is a strong howl that comes from the engine compartment (it like like the typicial intake sucking sound but ...the pitch is pretty high.) I changed the oil (royal purple is what I run) and changed the tranny fluid and filter (Toyota WS fluid with OEM filter). There was no change. I checked the fan cluth and it is fine. I also ran techron through the engine fuel system twice. I took it into my local dealership and that said it is my K&N filter. I am pretty sure it is not because it makes the same noise with the filter out. I have check everything and everything seems tight and the truck is actaully getting 18 mpg on the highway which is as good as it has every gotten. It has the sound regardless of the outside temp and regardless if the vehicle is warmed up. It does not make the noise when it is at normal idle...so I am at a loss as to what it is. Any suggestions of what to check would be much appreciated. Thank you in advance for any suggestions.

[dandersen]

I'm newly registered to the forum, but I have used it many times for excellent info to do all my own maintenance. I have a 2002 Sequoia 5.7L Limited w/ 162,000 miles that exhibits the same engine howl. I have done all the same fluid changes, etc. I have stock air filter. Engine and tranny seem tight, mileage is still good. Maybe once every ten days, the engine will stall at low idle. Other than that it runs strong. I have not yet taken it to the dealer to diagnose. I will be checking this post to see if anyone has insight as to the underlying issue. Thanks for starting the thread!