Flash's advice is dead on,
first, make sure the alternator override pulley is functioning correctly!!! IT is is easy to test; remove the serpentine drive belt and manually spin the alternator in the normal belt rotation direction by hand and then grab the drive pulley and abruptly stop it, it should allow the alternator internals to still spin for a brief moment. IF it does not, you need to replace the alternator drive pulley with a new one...
second, watch the serpentine belt tensioner belt pulley, if it jumps up and down a bunch with a good alternator drive pulley at idle, then the spring tensioner has become weak and the whole assembly needs to be replaced...
and thirdly, I discovered this on my own CRD that it seemed to rattle at idle with the AC on more than it should even after replacing both items listed on 1 & 2 above...
I ordered and replaced both engine motor mounts and the rattle at idle was cured, GONE!!!! The motor mounts are hydraulic internally and can cause rattling issues especially at idle....
The original drivers side motor mount after I removed it and shook it in my hand had an internal rattling sound....
