Why does the Convair 990 produce this howling sound during landing?
What you hear at 0:11-0:12 is just the sound of the engines being reduced to idle thrust, and the doppler effect as the aircraft passes your position. 

I have heard that sound hundreds(maybe thousands) of times. It is very normal, and sounds just like all jets of that generation.  

Here are more examples:

Skymaster DC-8-63F Landing

707 ! Touch &amp; Go, Depature, Landing. 

Boeing 707 Landing - Smokey!