[link to original on tumblr]
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So I listened for that when I listened to Pet Sounds this morning. It's in "I Know There's an Answer."
I've read a few things about Gershwin's influence on Wilson (and, of course, there's the Reimagines Gershwin album), so - while I haven't found anything to confirm it - I'd like to think that the inclusion of banjo in "I Know There's an Answer" is a slight nod to the Rhapsody in Blue.