What is that loud sucking sound? 

According to a Banco de Mexico study it is the sound of U.S. employers pulling workers up from Mexico.

Daniel Chiquiar and Alejandrina Salcedo of the Banco de Mexico present a study of the underlying economic factors and future immigration flow scenarios for Mexico to U.S. immigration.   They find evidence that of an across‐the‐board increase in the quantity of Mexican labor demanded in US, especially in construction, is a an important driving factor. Their study looks at the intensity of Mexican labor demand versus U.S. workers, i.e. the employment of Mexican immigrants relative to other U.S. workers in the industry.

They also provide projections of future Mexico-U.S. immigration.  Their work  suggests that for the 2011‐17 period, net inflows of all types of workers (legal and unauthorized) from Mexico to the United States could be on the order of 260,000 persons per year.   Their work suggest that future net inflow will be less than the net inflow of Mexicans during 1990‐2000, which was about 466,000 workers annually. If certain sectors in the U.S. such as construction were to experience large growth the net inflow could reach as much as 330,000 workers per year.