I have a 1999 F150 Lariat with the 4.6 and I love the truck, but I have noticed something. I have a whine that happens when I accelerate and goes away when I let off the accelerator. I have narrowed it to my dash in the center. I wonder if it is vacuum related or something else. Has anybody else have this issue?

 

My Expedition had more of a "whistle" to it, but it was this vacuum connector on the back of the climate control panel. Sorry I don't have a better pic. I couldn't get it to seal, so I actually took it off spit on the connector, LOL.... been good ever since.

 

My truck has the rotary controls. I am considering trying to retrofit one of those electronic temperature controls but that is a project down the road.

 

So does this

 

My Apologies, You were spot on...I popped off the environmental controls, took off the plug in question and noticed that there was a sticky residue. I used some RTV to create a seal. I took the truck for a drive after a few minutes and the sound is still there but lessened greatly. That tells me I am on the right track. I wonder if grease or glue will work better. Will continue to experiment. Thank you

 

Good luck! I hope it cooperates with you.