Calling HVAC professionals! Loud bang at Rheem condenser shutdown

One of my condensing units is making a very loud bang at shutdown that its identical twin does NOT make. Both units are Rheem RA1630 units, professionally installed on the same day in 6/2015. Both have Copeland scroll compressors. The affected unit has about 1/2 of the runtime hours of the unaffected unit - likely because it cools a smaller part of the house.


Both units have had multiple condenser fan motor failures (X-13 or ECM... can't remember) and were eventually converted over to PSC motors and have been trouble-free since then. I don't see any evidence of fan to shroud contact.


The unit starts fine, cools very well, but this noise is obviously troubling.



I assume the noise is coming from the compressor... is failure imminent?


I'd appreciate some feedback from you professionals out there!