gram and ga1279 I bet if the 2 of you were standing in front of a screw at shutdown you would both agree on what sounds normal and what sounds abnormal. It sounds like your interpreting what mlkwal1 is describing as a strange noise at shutdown differently. Just my unbiased opinion.
mlkwal1 you need to find some other screw machines and listen to them when the compressors cut off to get a better feel for what sounds normal. If in doubt I would rather replace the check than the compressor.