I searched for answers for an hour and I couldn't find the same issue.
I have a 99 SR5 w/ 112K - Timing belt done 3K ago with new belts all around.
Ok.... the truck is at idle (stop light / parked) when the idle will increase a little, I get a very weird rattle/knock but it goes away as soon as the idle drops back down ?????? It's not the belts, I can't pinpoint where it comes from. It seems to be towards the front of the engine bay. It disappears as soon as I gas it. It only happens when the idle jumps up a little.
Thanks in advance - Dave

 

on a 94 3vze the AS Reed valve IIRC caused the same symptoms... I know that doesnt help a whole lot because I'm not sure how much of the technology and the system carried over to the next generation of motors, but might be a good lead to look into.

 

Thanks for the tip - I will look into it. I threw in some Lucas Fuel Additive to see if it would help thinking it may be lifter oriented.

 

Is your idle speed set correctly?