Ford Repair: Whistling sound, ford f 350, k n air filter

Question
I recently purchased a 1997 Ford F 350 4x4 with the 460 efi gas engine and a K&N air filter.  After driving for a few miles (distance varies and I think it is temp related) the truck begins to make a sound similar to a stuck horn.  The sound goes away by applying slight pressure to the accelerator.  It also comes on while the vehicle is in motion and I release the gas pedal.  I have found that while the truck is making the noise if I remove the hose to the intake filter and plug it the noise stops.  There is a ball shaped hollow plastic device that this hose connects between the filter and the engine.  The truck runs great but the noise becomes extremely annoying.  Please help!

Answer
The ball-shaped piece is probably an "intake muffler" designed to keep the engine quiet.  It may be coming apart inside and need to be replaced, or the K&N could be causing problems.  I would try a new one and put a regular paper filter back in-- K&N's are bad for your vehicle anyway.