1999 454 high idle

I recently acquired a 1999 K-3500 with a 454 and manual transmission. I am experiencing DTC code P0507. Whenever I shift, the throttle hangs at about 1700 rpm with the clutch pushed down. Eventualy after a period of about three seconds it will begin to slow back down to around 1200 rpm. After a long time it may even settle down to 800rpm. It obviously shouldn't be hanging so long at these rpms. What do I need to do about this?

Usually due to a vacuum leak. Could be intake gasket, TB gasket or vacuum hose.

//2000CAYukon

So how does one go about finding a vacuum leak? Where do I start?

Use a spray can of carb or brake cleaner with the engine running. If there is a vacuum leak the rpms will change. Could also be the IAC valve, throttle linkage (binding), bad throttle body (binding).