I have a 2002 Volvo V70. It's turned into a long story but here are the main highlights. Any help, suggestions greatly appreciated. Good car, 135,000 miles, kept well, ran fine.

Then I heard a bizarre rattling noise when car stopped - not when running.
Had it towed. Mechanic said car was dead when they got it. Said rattling noise was Throttle. Replaced it. Then said car made a horrible noise. Determined it was out of power steering fluid. He said car good. Got it back and same initial noise still present. He said noise was power steering unit. He replaced that. I picked up car, he said noise was AC compressor. I decided to bring it to a different mechanic number. Their first mechanic said AC and or possibly the throttle wasn't programmed right. Their second mechanic said not the new throttle but thinks it's the alternator. If someone out there knows how a mechanic should diagnose if something is the alternator or AC, please let me know! Very frustrating