Whistle under the hood ???

Have 05 access cab with 2.7 and 4 speed automatic.

Problem:

There is a quiet intermittant whistle coming from under the hood. Almost like someone is whistling the second half of the "wolf-whistle." Seems more often when at moderate speed and suddenly let off the gas to slow down. On rare occasion it is when accelerating. I have taken off the secondary air filter. I have been under the hood to replace plugs and did not find any loose vacum lines and no accidental holes in the air filter box.
Does not seem to affect performance but just wonder if anything I should worry about.

 

well u covered what i was gonna say,, vaccum lines.. but maybe give them another check.. loose connection or cracked/slpit hose. other than that i dont know. sry dude and best of luck