Brief rapid clicking noise when engine first starts

My Lexus LS400 just started making a brief, rapid clicking noise like one of those electric screw drivers that has been set way down so that it won't strip the threads. It only lasts about 4-5 seconds and sounds like it's coming from near the fan. It only happens when the car has been sitting for at least a couple of days.

Any thoughts as to what it might be? I'd like to get some ideas before I take it in to be looked at.

Thanks, -J

It just happened again today. It happens just after the engine starts up or just as the engine starts up. It does not happen before it starts and it is not the same sound as I have heard in the past with a different car that had a weak battery or an alternator problem. The car starts right up without any problem or hesitation and then makes a very soft fairly rapid clicking sound that only lasts 2-3 seconds.

Lotta things can cause that. From here, no way to really pin it down. Remove the belt and see if it does it for starters. Or use a mechanics stethoscope to help pinning it down.