With that description, it sounds like the IWE is attempting to engage. Would make sense under hard acceleration as vacuum pressure would be the lowest then. The way to test this would be to do the same but with 4wd engaged. NO sound with 4wd engaged would lead me to believe that you have a vacuum leak. Just don't engage 4wd on pavement, as it could cause unnecessary bind.
__________________F150lifts BOSS. RAS. 5100s. 275/65/20 KO2. Rigid SAE fogs & SRM backups. CBIs in OEM HIDs
'13 F150 Platinum - Sterling Gray - 4wd - EB - 3.31 ELD - 7200lb GVWR
Kicker Substage. 35% fronts. Bedrug. Bakflip F1. EZ Down.