I got a call from the SO yesterday afternoon when she was driving back home complaining of a very loud noise and perceived loss of engine power. I told her to find a safe place to pull over and I'd come and investigate, I couldn't diagnose over the phone given the information I was given but I had torque converter/transmission failure running through my head. I drive the 40 miles to where she and the kids were and started the van. It sounded awful and the noise was there in while in P and Neutral. I opened the hood and crawled under the van when it was running but couldn't pinpoint where the noise was coming from with un-aided ears. I had her cycle the A/C on and off - BINGO - I could make the noise go away by turning the a/c ON. I thought I'd solved the issue and an a/c compressor clutch would fix this. I drove the van home without issue most of the way home, the noise came back upon decelerating for a stop light and again downshifting to go up a decent sized hill. I could get the noise to stop by turning off the a/c and back on both times. Got home and started reading the forums and found this about a similar sounding situation with a broken flex plate only his hammering noise occurred with the a/c OFF. I then went out and took the serpentine belt off to isolate the noise to the flex plate or the accessory drive. I could not replicate the noise with the serpentine belt off but did not run it very long since I didn't have a functional water pump. I tried spinning the alternator and a/c compressor pulleys and and noticed the alternator pulley and rotor were decoupled. Back to the forums and found out about the litens decoupler pulley on our vans. There is about 10deg of free rotation in the pulley before it engages the alternator rotor which doesn't seem right in my mechanical engineering mind (BSME) and after looking at the litens website have more concerns about the alternator pulley. Would cycling the a/c compressor cause the alternator pulley clutch to slip making a marbles in a tin can noise? I failed to mention that earlier this week we noticed the A/C wasn't blowing cold anymore either which lead me to my initial diagnosis of A/C compressor clutch failure. I'm currently unemployed and have to watch where I spend money, I hate to just start replacing parts with the hope of getting it right the first time. I do all my own wrenching unless expensive special tools are needed and I can't find one to borrow. If I have to replace the compressor I'll have to pay someone to recharge the system. I need to get this going and fast, we are supposed to be going to Minnesota next weekend for my brothers high school graduation party and I really would like to take the van.