Howling soung coming from intake manifold 2000 Ford EXP

I have a 2000 Ford Explorer, 4.0L V6, manual transmission, 125,000 miles. Purchased 2 years ago from private seller. In hot weather (never in temps below 70-75), there is a very loud howling sound coming from the intake manifold when sitting still, such as at a red light, or in a restaurant drive through. Today doing 75 on the freeway in 101 degree heat, rapidly lost speed from 75 to 60 with foot on gas. No power was lost to the radio, air cond. No engine light came on. Geared down from 5th to 4th and was able to recover speed. Exiting the freeway at a light, she stalled out. Started very easily, brought her home, removed the intake tube and started her up, the noise was still there, but not as loud. Revved the engine, then it just died. Started very easily again but still with the howling noise. The intake manifold looks dirty (carbon) but nothing too extreme, what are possible causes and solutions? Any help would be greatly appreciated.