I went into the crawlspace to investigate. Turns out the leaks are coming
out of the metal ducts' Y joints.

Here are some photos to illustrate:
http://www.smugmug.com/gallery/849974

I did not take photos of all the suspected leaks.

Although it may seem that the gap is small, but a lot of air is coming out.
What is the proper way to seal those joints? Some sort of sealant? I don't
want to use duct tape because they become dry and brittle after a few years
and then lose its effectiveness.