2000 Dodge Caravan Engine noise at idle

I have a 2000 grand Caravan, 3.3, 138,000 miles. The problem: When I start the van, there's a "grinding, ratcheting, clicking" sound (not high pitched). This is at times intermittent, it may come on and quickly fade, or it sometimes comes on and goes off and on. This noise/ sound gets louder if the A/C is turned on. It also only happens with the engine at idle. When driving, or when I depress the gas pedal, it goes away until the rpm's drop once again. Engine oil level is good, there's no sign of a blown head gasket. The most recent maintenance done on the van was the replacement of the a/c compressor (noise existed previously) and the alternator. Not sure what it is or could be. Any insight ill be greatly appreciatd.