cold start humming noise

i get a high pitch humming noise only on cold starts, even though the outside temps will be in the 80's. With a/c off, radio off, it lasts about 15 seconds, then I hear a loud crunch noise from engine area and the noise stops. 2013 Tacoma, 4 cyl, automatic, 11,800 miles. sound familiar? (It reminds me of a loud fuel pump on my old GM truck).

 

It's the air injection pump up by the air cleaner box. It blows fresh air into the exhaust manifold on cold starts. The excess oxygen causes the catalytic converter to heat up faster, which cuts cold-start emissions down.

Yes it is a funky noise, especially that pfft it makes when it shuts off