Slight engine rattle on throttle liftoff

[macro]

When I let off the throttle I can now hear a slight rattling, it sounds muffled though but definitely coming from the engine. Anyone have any ideas of what I should check? Any pulleys or perhaps the timing chain guide possible culprits? I just want to quickly figure out if it's going to turn into a major issue or not. Thanks!

[Rattlenbang]

When I've had something like that, it was RPM/load specific, which was pretty much a resonance issue. Any thin bit of metal, especially if loose, can do this. Exhaust is usual culprit. have someone play with throttle while you listen under hood.