Engine tapping noise

[SPLATT]

Last night I heard a loud noise coming from the engine compartment. The noise was so loud that it could be heard in the passenger compartment at highway speeds.
I pulled over, lifted the hood and found the noise coming from the top of engine, back near the firewall. The noise seems to be under the intake manifold.
The noise sounds just like a baseball card that has been placed in the spokes of a kid’s bicycle. The faster the rpm’s the faster the and louder the noise.
Any ideas before it goes to the dealer?
TH

[SPLATT]

I got lucky on this one. Took the truck to the dealer on the chance that by the time I pulled into the service bay the noise would be present.

As I entered the bay the noise magically appeared. Turns out, the purge valve on many of the Titans and Armadas is bad, well, kinda. The hoses that lead to and from the valve are not large enough to handle the volume of air it pumps, therefore it makes noise. The dealer assured me it is harmless and the fix is in the works. The fix should be larger hoses.

As of this writing Nissan owes me a stereo and purge valve.